docs/devel/qapi-code-gen: Turn FIXME admonitions into comments

docs/devel/qapi-code-gen: Turn FIXME admonitions into comments

We have two FIXME notes.  These FIXMEs are for QAPI developers.  They
are not useful for QAPI schema developers.  They are marked up as
admonitions, which makes them look important in generated HTML.

Turn them into comments.  QAPI developers will still see them (they
read and write the .rst).  QAPI schema developers may still see
them (if they read the .rst instead of the generated .html), but "this
is just for QAPI developers" should be more obvious.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230428105429.1687850-3-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>