mac_do(4): Revamp manual page after MAC/do updates

The new manual page in particular describes MAC/do's new rules syntax
and the jail support, as well as security considerations explaining the
overall design and how to leverage it in the most secure fashion.

Reviewed by: bapt, otis, Alexander Ziaee <concussious@runbox.com> (in part)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48153

(cherry picked from commit bc201841d13928c2a088fb07ac0a010b36eafa13)