loader.8: Document module commands

Begin documenting module loader commands in loader(8), where the reader
would look for them. Segregate them by their availability in individual
loader implementations, like build(7) targets.
These details were graciously donated by kevans.

Approved by: re (cperciva)
MFC after: 3 days
Reviewed by: kevans
Co-authored-by: Kyle Evans <kevans@freebsd.org>
Differential Revision: https://reviews.freebsd.org/D50727

(cherry picked from commit 6fb0d67819f730e8def56e139f780eff3c0cab0f)
(cherry picked from commit 513b9db1a829b4efba91c86f280ba97b26c82c1f)