docs/sphinx: adjust qapidoc to cope with same-line error sections

docs/sphinx: adjust qapidoc to cope with same-line error sections

Without this, the line the new QAPI doc generator chokes on

    # Errors: some

in doc-good.json.  We still use the old doc generator for the tests,
but we're about to correct that.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250618165353.1980365-2-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Fixes: e9fbf1a0c6c2 (docs/qapidoc: add visit_errors() method)
[Amend commit message to point to reproducer, and add Fixes:]
Signed-off-by: Markus Armbruster <armbru@redhat.com>