diff --git a/lib/includes/nghttp2/nghttp2.h b/lib/includes/nghttp2/nghttp2.h
index 32b9b2d6..16c9c042 100644
--- a/lib/includes/nghttp2/nghttp2.h
+++ b/lib/includes/nghttp2/nghttp2.h
@@ -2727,8 +2727,9 @@ NGHTTP2_EXTERN uint32_t
  *
  * Tells the |session| that |size| bytes for a stream denoted by
  * |stream_id| were consumed by application and are ready to
- * WINDOW_UPDATE.  This function is intended to be used without
- * automatic window update (see
+ * WINDOW_UPDATE.  The consumed bytes are counted towards both
+ * connection and stream level WINDOW_UPDATE.  This function is
+ * intended to be used without automatic window update (see
  * `nghttp2_option_set_no_auto_window_update()`).
  *
  * This function returns 0 if it succeeds, or one of the following
@@ -3390,6 +3391,9 @@ NGHTTP2_EXTERN int32_t
  * The |flags| is currently ignored and should be
  * :enum:`NGHTTP2_FLAG_NONE`.
  *
+ * The |stream_id| is the stream ID to send this WINDOW_UPDATE.  To
+ * send connection level WINDOW_UPDATE, specify 0 to |stream_id|.
+ *
  * If the |window_size_increment| is positive, the WINDOW_UPDATE with
  * that value as window_size_increment is queued.  If the
  * |window_size_increment| is larger than the received bytes from the