Do not mention loading modules via loader.conf

It is generally discouraged to load graphics kernel modules via
loader.conf(5). According to kevans@, loading Nvidia modules like this is
fragile for UEFI on FreeBSD <= 12.1. It might work, but those modules are
large and the loader's staging area is insufficient to contain kernel,
Nvidia modules, and much of anything else at the same time.

Also, while here, encourage people to use sysrc to modify the contents of
rc.conf(5).

Reviewed by: danfe
Approved by: maintainer (danfe)
Differential Revision: https://reviews.freebsd.org/D25506