hier: Document /usr/freebsd-dist

/usr/freebsd-dist is used used by various programs as the location for
FreeBSD distribution files. In-tree programs following this convention
are bsdinstall(8) and release(7).

Reviewed by: Pau Amma <pauamma@gundo.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34552