freebsd-base.7: Document the optional set

MFC after: 3 days
Reviewed by: ziaee, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D54065

(cherry picked from commit ce8dc5b12451e9922ab827dfe3d10abcd6e85165)