Document jail.conf Jail configuration

jail.conf is the preferred method of configuring jails in all supported
FreeBSD versions. Older rc.conf(5) configuration produces a warning.
Update the handbook example to match based on the submission in the PR,
with a few modifications.

PR: 187142
Submitted by: Jack-Benny Persson
Reviewed by: bcr, eadler
Approved by: bcr, eadler
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14524