docs/qapi-domain: add qapi:alternate directive

docs/qapi-domain: add qapi:alternate directive

Add the .. qapi:alternate:: directive, object, and qapi:alt:`name`
cross-reference role.

Add the "Alternatives:" field list for describing alternate choices. Like
other field lists that reference QAPI types, a forthcoming commit will
add cross-referencing support to this field.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-21-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>