qapi: Eliminate check_docs() and drop QAPIDoc.expr

Move what's left in check_docs() to check_expr().  Delegate the actual
checking to new QAPIDoc.check_expr().

QAPIDoc.expr is now unused; drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-39-git-send-email-armbru@redhat.com>
