Glib::RefPtr: minor release() improvements.
Glib::RefPtr: minor release() improvements.

https://bugzilla.gnome.org/show_bug.cgi?id=752812

    * glib/glibmm/refptr.h: add warning in a comment, replace
      warn_unused_result attribute with corresponding GLib macro.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm