Document the final heater control scheme (w/cmds, etc) for MOps Transit Operators

[zCoCo]

Incl. off-nominal cases (like forcing state) and changing source (with BIG warnings).
Test this documentation too to verify nothing is missing. Sometimes heater state had to be reset after being given a new power-level or threshold (can't recall which atm).

Include a warning about setting low power levels. NOTE from #196 :
Heater power level settings work now HOWEVER, if the power level is set to 10% (only level tested below 20%), the voltage measured won't go back up unless the power level is set to at least 70%. (that is, if you set the power level that low then want to go back up to some intermediate value, you must first set it to >=70%). The PWM waveforms never looked weird in this case though, so it's quite possible this was just a quirk of the cheap pokit DMM/OSC used to probe this system. As a rule of thumb, just don't go below 20% power.