Commit 57df0dff (qapi: Extend -compat to set policy for unstable
interfaces) neglected to update the "Limitation" paragraph to mention
feature 'unstable' in addition to feature 'deprecated'.  Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20231009110449.4015601-1-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>