docs/qapidoc: generate entries for undocumented members

docs/qapidoc: generate entries for undocumented members

Presently, we never have any empty text entries for members. The next
patch will explicitly generate such sections, so enable support for it
in advance.

The parser will generate placeholder sections to indicate undocumented
members, but it's the qapidoc generator that's responsible for deciding
what to do with that stub section.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-59-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
[Tweak the stub section text]
Signed-off-by: Markus Armbruster <armbru@redhat.com>