tools/extra_defs_gen/generate_extra_defs.cc: Update some comments
tools/extra_defs_gen/generate_extra_defs.cc: Update some comments

* tools/extra_defs_gen/generate_extra_defs.cc: Change comments stating
that get_properties() fails for GVolume. GVolume is an ordinary GInterface.
If the first call is get_properties(G_TYPE_VOLUME), it fails if glib
bug #465631 has not been fixed.

GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm