mitigations.7: markup functions with Fn macro

+ include this page in apropos Fn=memcpy

Reviewed by: emaste
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1428

(cherry picked from commit 22fb46af90d62d8177bfa0f1a8e4d89efa0b1905)