[layout] Update comment

src/hb-ot-map.hh

@@ -78,7 +78,8 @@ struct hb_ot_map_t
     }
   };
 
-  /* Pause functions return true if buffer (codepoint) content was changed. */
+  /* Pause functions return true if new glyph indices might have been
+   * added to the buffer.  This is used to update buffer digest. */
   typedef bool (*pause_func_t) (const struct hb_ot_shape_plan_t *plan, hb_font_t *font, hb_buffer_t *buffer);
 
   struct stage_map_t {