Add note to cbbackupmgr config and cbbackupmgr restore ref docs about special built-in Administrator not being backed up

https://docs.couchbase.com/server/current/backup-restore/cbbackupmgr-config.html

For --enable-users, note that the special built-in Administrator and its password is never backed up.

https://docs.couchbase.com/server/current/backup-restore/cbbackupmgr-restore.html

For --enable-users, note that the special built-in Administrator is never restored (since it's never backed up).  Also, note that for users to be restored, the backup repository must have been created using the --enable-users option in cbbackupmgr config.