doc: Reword the HPAC tutorial.

The overall meaning is the same. I added a paragraph to introduce nghttp2_hd_inflate_hd(), and added an explanation of using NGHTTP2_NV_FLAG_NO_INDEX for security sensitive headers.

doc/sources/tutorial-hpack.rst

 Tutorial: HPACK API
 ===================
 
-In this tutorial, we describe basic use of HPACK API in nghttp2
-library.  We briefly describe APIs for deflating and inflating header
-fields.  The example of using these APIs are presented as complete
-source code `deflate.c`_.
+In this tutorial, we describe basic use of nghttp2's HPACK API.  We
+briefly describe the APIs for deflating and inflating header fields.
+The full example of using these APIs, `deflate.c`_, is attached at the
+end of this page. It also resides in the examples directory in the
+archive or repository.
 
 Deflating (encoding) headers
 ----------------------------
 
-First we need to initialize :type:`nghttp2_hd_deflater` object using
-`nghttp2_hd_deflate_new()` function::
+First we need to initialize a :type:`nghttp2_hd_deflater` object using
+the `nghttp2_hd_deflate_new()` function::
 
     int nghttp2_hd_deflate_new(nghttp2_hd_deflater **deflater_ptr,
                                size_t deflate_hd_table_bufsize_max);
 
-This function allocates :type:`nghttp2_hd_deflater` object and
-initializes it and assigns its pointer to ``*deflater_ptr`` passed by
-parameter.  The *deflate_hd_table_bufsize_max* is the upper bound of
-header table size the deflater will use.  This will limit the memory
-usage in deflater object for dynamic header table.  If you doubt, just
+This function allocates a :type:`nghttp2_hd_deflater` object,
+initializes it, and assigns its pointer to ``*deflater_ptr``. The
+*deflate_hd_table_bufsize_max* is the upper bound of header table size
+the deflater will use.  This will limit the memory usage by the
+deflater object for the dynamic header table.  If in doubt, just
 specify 4096 here, which is the default upper bound of dynamic header
 table buffer size.
 
-To encode header fields, `nghttp2_hd_deflate_hd()` function::
+To encode header fields, use the `nghttp2_hd_deflate_hd()` function::
 
     ssize_t nghttp2_hd_deflate_hd(nghttp2_hd_deflater *deflater,
                                   uint8_t *buf, size_t buflen,
                                   const nghttp2_nv *nva, size_t nvlen);
 
 The *deflater* is the deflater object initialized by
-`nghttp2_hd_deflate_new()` function described above.  The *buf* is a
-pointer to buffer to store encoded byte string.  The *buflen* is
-capacity of *buf*.  The *nva* is a pointer to :type:`nghttp2_nv`,
-which is an array of header fields to deflate.  The *nvlen* is the
-number of header fields which *nva* contains.
+`nghttp2_hd_deflate_new()` described above. The encoded byte string is
+written to the buffer *buf*, which has length *buflen*.  The *nva* is
+a pointer to an array of headers fields, each of type
+:type:`nghttp2_nv`.  *nvlen* is the number of header fields which
+*nva* contains.
 
 It is important to initialize and assign all members of
-:type:`nghttp2_nv`.  If a header field should not be inserted in
-dynamic header table for a security reason, set
-:macro:`NGHTTP2_NV_FLAG_NO_INDEX` flag in :member:`nghttp2_nv.flags`.
+:type:`nghttp2_nv`. For security sensitive header fields (such as
+cookies), set the :macro:`NGHTTP2_NV_FLAG_NO_INDEX` flag in
+:member:`nghttp2_nv.flags`.  Setting this flag prevents recovery of
+sensitive header fields by compression based attacks: This is achieved
+by not inserting the header field into the dynamic header table.
 
 `nghttp2_hd_deflate_hd()` processes all headers given in *nva*.  The
 *nva* must include all request or response header fields to be sent in
 one HEADERS (or optionally following (multiple) CONTINUATION
 frame(s)).  The *buf* must have enough space to store the encoded
-result.  Otherwise, the function will fail.  To estimate the upper
-bound of encoded result, use `nghttp2_hd_deflate_bound()` function::
+result, otherwise the function will fail.  To estimate the upper bound
+of the encoded result length, use `nghttp2_hd_deflate_bound()`::
 
     size_t nghttp2_hd_deflate_bound(nghttp2_hd_deflater *deflater,
                                     const nghttp2_nv *nva, size_t nvlen);
 
-Pass this function with the same parameters *deflater*, *nva* and
-*nvlen* which will be passed to `nghttp2_hd_deflate_hd()`.
+Pass this function the same parameters (*deflater*, *nva*, and
+*nvlen*) which will be passed to `nghttp2_hd_deflate_hd()`.
 
-The subsequent call of `nghttp2_hd_deflate_hd()` will use current
-encoder state and perform differential encoding which is the
-fundamental compression gain for HPACK.
+Subsequent calls to `nghttp2_hd_deflate_hd()` will use the current
+encoder state and perform differential encoding, which yields HPAC's
+fundamental compression gain.
 
-Once `nghttp2_hd_deflate_hd()` fails, it cannot be undone and its
-further call with the same deflater object shall fail.  So it is very
-important to use `nghttp2_hd_deflate_bound()` to know the required
-size of buffer.
+If `nghttp2_hd_deflate_hd()` fails, the failure is fatal and any
+further calls with the same deflater object will fail.  Thus it's very
+important to use `nghttp2_hd_deflate_bound()` to determine the
+required size of the output buffer.
 
-To delete :type:`nghttp2_hd_deflater` object, use `nghttp2_hd_deflate_del()`
-function.
+To delete a :type:`nghttp2_hd_deflater` object, use the
+`nghttp2_hd_deflate_del()` function.
 
 Inflating (decoding) headers
 ----------------------------
 
-We use :type:`nghttp2_hd_inflater` object to inflate compressed header
-data.  To initialize the object, use `nghttp2_hd_inflate_new()`::
+A :type:`nghttp2_hd_inflater` object is used to inflate compressed
+header data.  To initialize the object, use
+`nghttp2_hd_inflate_new()`::
 
     int nghttp2_hd_inflate_new(nghttp2_hd_inflater **inflater_ptr);
 
-To inflate header data, use `nghttp2_hd_inflate_hd()` function::
+To inflate header data, use `nghttp2_hd_inflate_hd()`::
 
     ssize_t nghttp2_hd_inflate_hd(nghttp2_hd_inflater *inflater,
                                   nghttp2_nv *nv_out, int *inflate_flags,
                                   uint8_t *in, size_t inlen, int in_final);
 
+`nghttp2_hd_inflate_hd()` reads a stream of bytes and outputs a single
+header field at a time. Multiple calls are normally required to read a
+full stream of bytes and output all of the header fields.
+
 The *inflater* is the inflater object initialized above.  The *nv_out*
-is a pointer to :type:`nghttp2_nv` to store the result.  The *in* is a
-pointer to input data and *inlen* is its length.  The caller is not
-required to specify whole deflated header data to *in* at once.  It
-can call this function multiple times for portion of the data in
-streaming way.  If *in_final* is nonzero, it tells the function that
-the passed data is the final sequence of deflated header data.  The
-*inflate_flags* is output parameter and successful call of this
-function stores a set of flags in it.  It will be described later.
+is a pointer to a :type:`nghttp2_nv` into which one header field may
+be stored.  The *in* is a pointer to input data, and *inlen* is its
+length.  The caller is not required to specify the whole deflated
+header data via *in* at once: Instead it can call this function
+multiple times as additional data bytes become available.  If
+*in_final* is nonzero, it tells the function that the passed data is
+the final sequence of deflated header data.
+
+The *inflate_flags* is an output parameter; on success the function
+sets it to a bitset of flags.  It will be described later.
 
 This function returns when each header field is inflated.  When this
-happens, the function sets :macro:`NGHTTP2_HD_INFLATE_EMIT` flag to
-*inflate_flag* parameter and header field is stored in *nv_out*.  The
-return value indicates the number of data read from *in* to processed
-so far.  It may be less than *inlen*.  The caller should call the
-function repeatedly until all data are processed by adjusting *in* and
-*inlen* with the processed bytes.
+happens, the function sets the :macro:`NGHTTP2_HD_INFLATE_EMIT` flag
+in *inflate_flags*, and a header field is stored in *nv_out*.  The
+return value indicates the number of bytes read from *in* processed so
+far, which may be less than *inlen*.  The caller should call the
+function repeatedly until all bytes are processed. Processed bytes
+should be removed from *in*, and *inlen* should be adjusted
+appropriately.
 
 If *in_final* is nonzero and all given data was processed, the
-function sets :macro:`NGHTTP2_HD_INFLATE_FINAL` flag to
-*inflate_flag*.  If the caller sees this flag set, call
+function sets the :macro:`NGHTTP2_HD_INFLATE_FINAL` flag in
+*inflate_flags*.  When you see this flag set, call the
 `nghttp2_hd_inflate_end_headers()` function.
 
-If *in_final* is zero and :macro:`NGHTTP2_HD_INFLATE_EMIT` flag is not
-set, it indicates that all given data was processed.  The caller is
-required to pass subsequent data.
+If *in_final* is zero and the :macro:`NGHTTP2_HD_INFLATE_EMIT` flag is
+not set, it indicates that all given data was processed.  The caller
+is required to pass additional data.
 
 It is important to note that the function may produce one or more
 header fields even if *inlen* is 0 when *in_final* is nonzero, due to
 differential encoding.
 
-The example use of `nghttp2_hd_inflate_hd()` is shown in
+Example usage of `nghttp2_hd_inflate_hd()` is shown in the
 `inflate_header_block()` function in `deflate.c`_.
 
-To delete :type:`nghttp2_hd_inflater` object, use `nghttp2_hd_inflate_del()`
-function.
+Finally, to delete a :type:`nghttp2_hd_inflater` object, use
+`nghttp2_hd_inflate_del()`.