MFC r325441:

Extend the synopsis section of md(4) to look more like other manpages
of this kind. Describe how to compile the driver into the kernel
and how to load it as a module.
This is useful for people using the MINIMAL kernel configuration file.

PR: 218610
Submitted by: Harald Schmalzbauer (bugzilla.freebsd@omnilan.de)
Reviewed by: noone (1 month inactivity)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12271