[glibmm] Add a comment

[Murray Cumming <murrayc src gnome org>]

commit 0600b42dc458993c83e16956c509917d9275e87c
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Sep 26 21:59:51 2013 +0200

    Add a comment

 gio/src/actionmap.hg |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gio/src/actionmap.hg b/gio/src/actionmap.hg
index 8d17aeb..97cf32a 100644
--- a/gio/src/actionmap.hg
+++ b/gio/src/actionmap.hg
@@ -190,6 +190,7 @@ public:
   _WRAP_VFUNC(Glib::RefPtr<Action> lookup_action(const Glib::ustring& name) const, "lookup_action")
 
   //TODO: Change this to use const & when we can break ABI.
+  // ( Changing it causes a symbol lookup error when trying to run already-built applications. )
 #m4 _CONVERSION(`GAction*', `Glib::RefPtr<Action>', `Glib::wrap($3, true)')
   _WRAP_VFUNC(void add_action(Glib::RefPtr<Action> action) const, "add_action")
   _WRAP_VFUNC(void remove_action(const Glib::ustring& name), "remove_action")