Update doc

lib/nghttp2_buf.h

@@ -138,7 +138,9 @@ typedef struct {
   nghttp2_buf_chain *cur;
   /* Memory allocator */
   nghttp2_mem *mem;
-  /* The buffer capacity of each buf */
+  /* The buffer capacity of each buf.  This field may be 0 if
+     nghttp2_bufs is initialized by nghttp2_bufs_wrap_init* family
+     functions. */
   size_t chunk_length;
   /* The maximum number of nghttp2_buf_chain */
   size_t max_chunk;
@@ -201,6 +203,9 @@ void nghttp2_bufs_free(nghttp2_bufs *bufs);
  * words, max_chunk = chunk_keep = 1.  To free the resource allocated
  * for |bufs|, use nghttp2_bufs_wrap_free().
  *
+ * Don't use the function which performs allocation, such as
+ * nghttp2_bufs_realloc().
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *
@@ -217,6 +222,9 @@ int nghttp2_bufs_wrap_init(nghttp2_bufs *bufs, uint8_t *begin, size_t len,
  * free the resource allocated for |bufs|, use
  * nghttp2_bufs_wrap_free().
  *
+ * Don't use the function which performs allocation, such as
+ * nghttp2_bufs_realloc().
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *