Revision 51461

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