jail: document the mac.label parameter

In particular, we should provide a hint about mac.conf(5), since libjail
will just use the mac_prepare_type(3) API to provide a reasonably sane
interface for system administrators. Progammers wanting to fetch an
arbitrary MAC label would need to bypass libjail and use jail_get(2)
directly with their own prepared struct mac.

Differential Revision: https://reviews.freebsd.org/D54067