13.1 relnotes: Add sections derived from 13.1 RELNOTES file.

[karels]

Summary

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.

Test Plan

reviewed local build

[pauamma_gundo.com]

Minor correction, can be fixed before committing.

website/content/en/releases/13.1R/relnotes.adoc

109

Much more prevalent in US English.