GDBusConnection: Document memory management semantics for get_property()

Turns out we are leaking non-floating GVariant instances returned by
get_property() functions.

Also avoid imprecise language such as "newly-allocated GVariant" as
this doesn't specify whether the variant can be floating or not.

Also see https://bugzilla.gnome.org/show_bug.cgi?id=627974

 as it is
very related to this change.

Signed-off-by: David Zeuthen <davidz@redhat.com>