Short version:
Try running 'man mdadm.conf'  If that doesn't produce a manual page
please follow this link:

http://man-wiki.net/index.php/5:mdadm.conf

Long version:
Rescue is probably the name of the last system to update the
meta-data, either a rescue cd / system used for recovery or setup.

The normal mdadm manual page should elaborate the use of these options:

       -U, --update=homehost
       --auto-update-homehost