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>
12 files changed