tests/qapi-schema/doc-bad-section: New, factored out of doc-good

A negative test case crept into doc-good.json: invalid use of section
markup we currently fail to reject.  Move this into its own
doc-bad-section.json.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002141341.24616-6-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
