[gtk+] GtkEventController: Add some comment describing the filter_event private vfunc

[Carlos Garnacho <carlosg src gnome org>]

commit b24a865d9bcc7f716a81fd218a3b637faee13f0e
Author: Carlos Garnacho <carlosg gnome org>
Date:   Tue Aug 23 20:31:29 2016 +0200

    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 |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkeventcontrollerprivate.h b/gtk/gtkeventcontrollerprivate.h
index 2f66ecc..f847198 100644
--- a/gtk/gtkeventcontrollerprivate.h
+++ b/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];