qapi: Move empty doc section checking to doc parser

Results in a more precise error location, but the real reason is
emptying out check_docs() step by step.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-35-git-send-email-armbru@redhat.com>
