qapi: Belatedly adjust limitations documentation

qapi: Belatedly adjust limitations documentation

Commit 57df0dff1a "qapi: Extend -compat to set policy for unstable
interfaces" (v6.2.0) took care of covering experimental features, but
neglected to adjust a comment suggesting to cover it.  Adjust it now.

Fixes: 57df0dff1a1f4c846aa74a082bfd595a8a990015
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220225084538.218876-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>