Document spindown constraints as given in the original commit message[1] and later clarification provided by phk.
Document spindown constraints as given in the original commit
message[1] and later clarification provided by phk.

[1] http://docs.freebsd.org/cgi/mid.cgi?200803171033.m2HAXOeN055116

Reviewed by:	brueffer, phk, ed

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