qapi: Document QCryptodevBackendServiceType

qapi: Document QCryptodevBackendServiceType

QCryptodevBackendServiceType was introduced by
bc304a6442e (cryptodev: Introduce server type in QAPI). However there
is a lack of member description.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Drop QCryptodevBackendServiceType from pragma documentation-exceptions]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240911112545.2248881-9-armbru@redhat.com>