md(4): improve documentation of preloading

It's not immediately clear by what mechanism loader(8) will be loading the
preloaded file. Specifically name-drop loader.conf(5) with a pointer to the
module loading section and a description of what the 'name' should look
like, because that certainly isn't clear from the loader.conf(5) standpoint.

The default loader.conf already has a pointer to md(4) where it appears and
the reference to loader.conf in the new version of this manpage should make
it more clear that this is where one should look for information.

Reported by: swills
Reviewed by: swills, manpages (bcr)
With revision by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D22844