doc: add item for "-M enforce-config-section"

It's never documented, and now we have one more parameter for it (which
obsoletes this one). Document it properly.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1499396048-21657-1-git-send-email-peterx@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Removed 'Although now' commit message as per Eduardo's review