14.0 relnotes: some additions

I did a pass of the current release notes, fixing some very minor issues
I saw and adding things I know to be missing, some of it being my own
work.

The biggest part of the change is introduction of a new "Documentation"
section to describe man page changes.

Reviewed by: karels, carlavilla, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42496