Update doc

lib/includes/nghttp2/nghttp2.h

@@ -2629,14 +2629,20 @@ NGHTTP2_EXTERN void nghttp2_session_del(nghttp2_session *session);
  *
  * 6. :type:`nghttp2_before_frame_send_callback` is invoked.
  *
- * 7. :type:`nghttp2_send_callback` is invoked one or more times to
+ * 7. If :enum:`NGHTTP2_ERR_CANCEL` is returned from
+ *    :type:`nghttp2_before_frame_send_callback`, the current frame
+ *    transmission is canceled, and
+ *    :type:`nghttp2_on_frame_not_send_callback` is invoked.  Abort
+ *    the following steps.
+ *
+ * 8. :type:`nghttp2_send_callback` is invoked one or more times to
  *    send the frame.
  *
- * 8. :type:`nghttp2_on_frame_send_callback` is invoked.
+ * 9. :type:`nghttp2_on_frame_send_callback` is invoked.
  *
- * 9. If the transmission of the frame triggers closure of the stream,
- *    the stream is closed and
- *    :type:`nghttp2_on_stream_close_callback` is invoked.
+ * 10. If the transmission of the frame triggers closure of the
+ *     stream, the stream is closed and
+ *     :type:`nghttp2_on_stream_close_callback` is invoked.
  *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes: