rc.subr(8): Document when required_* variables are processed

At the moment rc.subr(8) supports the following required_* variables:
required_dirs, required_files, required_modules and required_vars.

This patch documents when every of those required_* variables is actually
processed (before or after running start_precmd).

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Sponsored by: Bally Wulff Games & Entertainment GmbH
Differential Revision: https://reviews.freebsd.org/D17895