Document a couple of build knobs
Document a couple of build knobs

PR:		misc/71032 (partly)
Submitted by:	Divacky Roman <xdivac02@stud.fit.vutbr.cz>
Reviewed by:	ru
MFC after:	3 days

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