Document /boot/mbr (non-interactive boot block)
Document /boot/mbr (non-interactive boot block)

PR:		doc/111840
Submitted by:	Warren Block <wblock@wonkity.com>
MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase