qapi: Improve error message for empty doc sections

qapi: Improve error message for empty doc sections

Improve the message for an empty tagged section from

    empty doc section 'Note'

to

    text required after 'Note:'

and the message for an empty argument or feature description from

    empty doc section 'foo'

to

    text required after '@foo:'

Improve the error position to refer to the beginning of the empty
section instead of its end.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240216145841.2099240-8-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>