[gtkmm] Add a comment

[Murray Cumming <murrayc src gnome org>]

commit 98d8cf54a17f75e117a1043624f23c261fd1df9e
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Oct 11 11:05:54 2013 +0200

    Add a comment

 gtk/src/listbox.hg |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtk/src/listbox.hg b/gtk/src/listbox.hg
index 09ec21c..3968e1f 100644
--- a/gtk/src/listbox.hg
+++ b/gtk/src/listbox.hg
@@ -97,6 +97,7 @@ public:
   _WRAP_METHOD(void insert(Widget& child, int position), gtk_list_box_insert)
 
   //We added this in the gtkmm API, because this is clearer than the magic -1 value for position with 
insert().
+  //The C API developers disagree: https://bugzilla.gnome.org/show_bug.cgi?id=705558#c12
   /** Append a widget to the list. If a sort function is set, the widget will
    * actually be inserted at the calculated position and this function has the
    * same effect of Gtk::Container::add().