13.2R relnotes: numerous additions plus edits

Add entries for many remaining "Relnotes: yes" entries in the git
log. Add solicited input in many areas. Some additional edits
to previous entries, and add a few details. Strengthen recommendation
to reboot after installing user software. Format filenames with
[.filename]#name#. Updated sample entry with gitref reference.

Approved by: delphij
Reviewed by: pauamma, delphij
Differential Revision: https://reviews.freebsd.org/D38639