pam_xdg.8: Improve usage and standards

Explain the directory created by this module,
its usage, and a bit more about the standard.

MFC after: 3 days
Reviewed by: mhorne, manu
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D50440

(cherry picked from commit 616bb0b230835e8b88d032e2026854ad031afea2)