docs/qapi-domain: add return-nodesc

docs/qapi-domain: add return-nodesc

This form is used to annotate a return type without an accompanying
description, for when there is no "Returns:" information in the source
doc, but we have a return type we want to generate a cross-reference to.

The syntax is:

:return-nodesc: TypeName

It's primarily necessary because Sphinx always expects both a type and a
description for the prior form and will format it accordingly. To have a
reasonable rendering when the body is missing, we need to use a
different info field list entirely.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250711051045.51110-2-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
[Long line wrapped]
Signed-off-by: Markus Armbruster <armbru@redhat.com>