Add section about automatic runtime directories handling since FreeBSD 14.1

FreeBSD 14.1 added functionality to automatically create XDG runtime
directories needed for Wayland to run. Add a section describing this
and how to handle it in pre-14.1 releases.
I changed the resulting diff to use active voice in the last sentence.

Reported by: concussious.bugzilla_runbox.com
Approved by: manu, lwhsu,x11
Differential Revision: https://reviews.freebsd.org/D45953