kldload(8): Improve phrasing

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