clarify SmartCharging API doc (#310)

src/MicroOcpp.h

@@ -276,8 +276,10 @@ void setEnergyMeterInput(std::function<int()> energyInput, unsigned int connecto
 
 void setPowerMeterInput(std::function<float()> powerInput, unsigned int connectorId = 1); //Input of the power meter reading in W
 
-//Smart Charging Output, alternative for Watts only, Current only, or Watts x Current x numberPhases. Only one
-//of them can be set at a time
+//Smart Charging Output, alternative for Watts only, Current only, or Watts x Current x numberPhases.
+//Only one of the Smart Charging Outputs can be set at a time.
+//MO will execute the callback whenever the OCPP charging limit changes and will pass the limit for now
+//to the callback. If OCPP does not define a limit, then MO passes the value -1 for "undefined".
 void setSmartChargingPowerOutput(std::function<void(float)> chargingLimitOutput, unsigned int connectorId = 1); //Output (in Watts) for the Smart Charging limit
 void setSmartChargingCurrentOutput(std::function<void(float)> chargingLimitOutput, unsigned int connectorId = 1); //Output (in Amps) for the Smart Charging limit
 void setSmartChargingOutput(std::function<void(float,float,int)> chargingLimitOutput, unsigned int connectorId = 1); //Output (in Watts, Amps, numberPhases) for the Smart Charging limit