linux(4): man page tweaks

Advertise rc.conf method as the right way to enable it, mention
file system mapping... and change some wording.

Reviewed By: emaste, debdrup, Pau Amma
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D33720

(cherry picked from commit bd8077ee84bd990d88607cc13f36f3da2a4dce10)