doc: update TYPE_MIGRATION documents

[Peter collected Eduardo's patch comment and formatted into patch]

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1499242883-2184-5-git-send-email-peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>