kldload(8): Improve phrasing

Mountroot isn't documented in the extant manual pages - so this
phrasing, while less absolute and concise, still conveys which
modules are recommended to be handled via loader.conf(5), and it also
does a better job of elucidating that the modules can include filesystem
drivers.

Submitted by: kevans (earlier version)
Reported by: imp, kevans, eugen
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D28542