[libsigc++2] trackable: Add a comment

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 00d7c733fafd737d42747a01483ed304cce61e90
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Sat Oct 31 09:57:06 2015 +0100

    trackable: Add a comment

 sigc++/trackable.cc |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/trackable.cc b/sigc++/trackable.cc
index 5702b99..385f97c 100644
--- a/sigc++/trackable.cc
+++ b/sigc++/trackable.cc
@@ -37,9 +37,12 @@ trackable::trackable(const trackable& /*src*/)
 : callback_list_(nullptr)
 {}
 
-// Don't copy the notification list.
+// Don't move the notification list.
 // The objects watching src don't need to be notified when the new object dies.
 // They need to be notified now, because src probably becomes useless.
+//
+// If trackable's move constructor is modified, check if Glib::Object's
+// move constructor should be modified similarly.
 trackable::trackable(trackable&& src) noexcept
 : callback_list_(nullptr)
 {