Commit 57df0dff "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: 57df0dff
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>