docs/qapi-domain: add :namespace: override option

docs/qapi-domain: add :namespace: override option

Akin to the :module: override option, the :namespace: options allows you
to forcibly override the contextual namespace associatied with a
definition.

We don't necessarily actually need this, but I felt compelled to stick
close to how the Python domain works that offers context overrides.

As of this commit, it is possible to add e.g. ":namespace: QMP" to any
QAPI directive to forcibly associate that definition with a given
namespace.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250313044312.189276-5-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>