mac(9): update SEE ALSO

Rather than maintaining an incomplete list of MAC modules references,
just reference mac(4), where such a list can be found.

Reviewed by: Mina Galić <freebsd@igalic.co>
Reviewed by: Pau Amma <pauamma@gundo.com>
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40485

(cherry picked from commit 0449bc249b78a8c85afe375e581432351f90427d)