qapi: rephrase return docs to avoid type name

qapi: rephrase return docs to avoid type name

"Returns: <description>" is rendered like "Return: <Type> –
<description>".  Mentioning the type in the description again is
commonly redundant.  Rephrase such descriptions not to.

Well, I tried. Maybe not very hard. Sorry!

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250711051045.51110-5-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
[Commit message amended to explain why]
Signed-off-by: Markus Armbruster <armbru@redhat.com>