Update doc

lib/includes/nghttp2/nghttp2.h

@@ -870,6 +870,11 @@ typedef ssize_t (*nghttp2_recv_callback)
  * argument passed in to the call to `nghttp2_session_client_new()` or
  * `nghttp2_session_server_new()`.
  *
+ * If frame is HEADERS or PUSH_PROMISE, the ``nva`` and ``nvlen``
+ * member of their data structure are always ``NULL`` and 0
+ * respectively. The header name/value pairs are emitted via
+ * :type:`nghttp2_on_header_callback`.
+ *
  * The implementation of this function must return 0 if it
  * succeeds. If nonzero value is returned, it is treated as fatal
  * error and `nghttp2_session_recv()` and `nghttp2_session_mem_recv()`
@@ -889,6 +894,14 @@ typedef int (*nghttp2_on_frame_recv_callback)
  * third argument passed in to the call to
  * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
  *
+ * If frame is HEADERS or PUSH_PROMISE, the ``nva`` and ``nvlen``
+ * member of their data structure are always ``NULL`` and 0
+ * respectively.
+ *
+ * If this callback is called, :type:`nghttp2_on_header_callback` and
+ * :type:`nghttp2_on_end_headers_callback` will not be called for this
+ * frame.
+ *
  * The implementation of this function must return 0 if it
  * succeeds. If nonzero is returned, it is treated as fatal error and
  * `nghttp2_session_recv()` and `nghttp2_session_send()` functions
@@ -1154,6 +1167,10 @@ typedef int (*nghttp2_on_header_callback)
  * will handle the error by either transmitting RST_STREAM or GOAWAY
  * and terminate session.
  *
+ * If the |error_code| is not :enum:`NGHTTP2_NO_ERROR`, then
+ * :type:`nghttp2_on_request_recv_callback` will not called for this
+ * frame if the |frame| is HEADERS.
+ *
  * The implementation of this function must return 0 if it
  * succeeds. If nonzero value is returned, it is treated as fatal
  * error and `nghttp2_session_recv()` and `nghttp2_session_mem_recv()`
@@ -1500,7 +1517,12 @@ int nghttp2_session_send(nghttp2_session *session);
  *       one or more times to receive whole frame.
  *    2. If the received frame is valid,
  *       :member:`nghttp2_session_callbacks.on_frame_recv_callback` is
- *       invoked.  If the frame is the final frame of the request,
+ *       invoked. If frame is either HEADERS or PUSH_PROMISE,
+ *       :member:`nghttp2_session_callbacks.on_header_callback` is
+ *       invoked for each header name/value pair. After all name/value
+ *       pairs are emitted (or decompression failed),
+ *       :member:`nghttp2_session_callbacks.on_end_headers_callback`
+ *       is invoked. If the frame is the final frame of the request,
  *       :member:`nghttp2_session_callbacks.on_request_recv_callback`
  *       is invoked.  If the reception of the frame triggers the
  *       closure of the stream,