MFC: r180005, cvs rev. 1.31 atacontrol.8
MFC: r180005, cvs rev. 1.31 atacontrol.8

  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

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