[gtkmm] Add a comment

[Murray Cumming <murrayc src gnome org>]

commit ce9ce0292705d4463ec30caa3c6068e491fd5838
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Sep 30 12:26:57 2013 +0200

    Add a comment

 gtk/src/menubar.hg |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtk/src/menubar.hg b/gtk/src/menubar.hg
index 5b848fe..45e79e6 100644
--- a/gtk/src/menubar.hg
+++ b/gtk/src/menubar.hg
@@ -45,6 +45,7 @@ public:
 
   //This is custom-implemented because the gtk_menu_new_from_model() does more
   //than just call g_object_new. See https://bugzilla.gnome.org/show_bug.cgi?id=704671
+  //This might be nearly-useless. See https://bugzilla.gnome.org/show_bug.cgi?id=708905
   /** Creates a new MenuBar and populates it with menu items
    * and submenus according to the @a model.
    *