qapi: lift restriction on using '=' in doc blocks

qapi: lift restriction on using '=' in doc blocks

We reject lines starting with '=' in definition documentation.  This
made sense when such lines were headings in free-form documentation,
but not in definition documentation.

Before the previous commit, lines starting with '=' were headings in
free-form documentation, and rejected in definition documentation,
where such headings could not work.

The previous commit dropped the headings feature from free-form
documentation, because we can simply use plain rST headings.
Rejecting them in definition documentation no longer makes sense, so
drop that, too.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250618165353.1980365-6-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Amend commit message to explain why]
Signed-off-by: Markus Armbruster <armbru@redhat.com>