update/correct the Handbook Diskless operations chapter

• Add the -maproot=root to the NFS export configuration
• Mention base archive from the ftp server as an alternative way to install the base system
• Fix the <command> usage
• Add details about how to use md_size files to increase the default mfs size for /var and /etc

PR: 213175
Submitted by: Natacha Porte <natbsd@instinctive.eu>
Reviewed by: sevan bcr ian
Approved by: ian sevan
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14336