qapi: Add missing union tag documentation Low-hanging fruit, and except for StatsFilter, the only members of these unions lacking documentation. Signed-off-by: Markus Armbruster <armbru@redhat.com> Message-ID: <20240205074709.3613229-16-armbru@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>