nfsv4.4: Document setup of a NFSv4 root fs

Commit 8b9775912cbc added support for an NFSv4 mounted
root file system. This patch documents how to set this
up. It also includes some minor updates and fixes
some formatting.

This is a content change.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56317
Fixes: 8b9775912cbc ("nfs_diskless: Add support for an NFSv4 root fs")