GtkEventController: Add some comment describing the filter_event private vfunc

It might not be entirely clear what the boolean return value means.

https://bugzilla.gnome.org/show_bug.cgi?id=770026

gtk/gtkeventcontrollerprivate.h

@@ -37,6 +37,9 @@ struct _GtkEventControllerClass
 
   /*<private>*/
 
+  /* Tells whether the event is filtered out, %TRUE makes
+   * the event unseen by the handle_event vfunc.
+   */
   gboolean (* filter_event) (GtkEventController *controller,
                              const GdkEvent     *event);
   gpointer padding[10];