protect.1: document existence of _oomprotect

Improve discoverability of the functionality by mentioning in the
userland tool manual. Add a SEE ALSO entry to rc.conf(5) where more
details are provided.

Sponsored by: Fudo Security (a.wolk)
Differential Revision: https://reviews.freebsd.org/D30334

(cherry picked from commit c8b6be0f7d1b92d11b279761685f61f6702700a1)