13.1 relnotes: Add sections derived from 13.1 RELNOTES file.

Incorporate portions of the 13.1 RELNOTES file not in 13.0, rephrase
without the imperative "Add ..." first lines, and add markdown.
NFS section reviewed by Rick Macklem; One True Awk reviewed by Warner
Losh.

Reviewed by: gjb, pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D34864