Properly document invoking MIT Kerberos daemons through rc.conf.
This commit chases r270782 in base which renamed the kerberos5_*
varibales to their current form.

PR: 197337
Reported by: Adam McDougall <ebay at looksharp.net>
Reviewed by: delphij, bcr
Approved by: delphij, bcr
Differential Revision: https://reviews.freebsd.org/D24253