rc.conf: Document zfskeys

Fixes: 33ff39796ffe Add zfskeys rc.d script for auto-loading encryption keys
MFC after: 3 days
Reviewed by: allanjude
Sponsored by: Modirum
Sponsored by: Klara, Inc
Differential Revision: https://reviews.freebsd.org/D34427

(cherry picked from commit 8719e8a951b78ca555ed777e99d5e2b90f3c4e7b)