docs/qapidoc: support header-less freeform sections

docs/qapidoc: support header-less freeform sections

The code as written crashes when a free-form documentation block doesn't
start with a heading or subheading, for example:

| ##
| # Just text, no heading.
| ##

The code will attempt to use the `node` variable uninitialized. To fix,
create a generic block to insert the doc text into.

(This patch also removes a lingering pylint warning in the QAPIDoc
implementation that prevents getting a clean baseline to use for
forthcoming additions.)

Fixes: 43e0d14ee09a (docs/sphinx: fix extra stuff in TOC after freeform QMP sections)
Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250224033741.222749-5-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Test updated to cover this]
Signed-off-by: Markus Armbruster <armbru@redhat.com>