kldload(8): Improve phrasing

[debdrup]

Summary

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@, Eugene Grosbein <eugen at grosbein.net>

Test Plan

mandoc and igor

[kevans]

I could probably bikeshed this all day just by myself, but this is a substantial improvement -- it's still somewhat strong wording ("Only those ... should be handled by loader.conf(5)"), but IMO it includes a good enough example of the set of modules it's talking about for one to naturally exercise caution when moving stuff from loader.conf(5) to kld_list and those of us that know better will ignore the strength of the message anyways.