chore: clarify use of the engine state variables for sound triggers

Author: YoRyan

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/PowerSupplies/DieselPowerSupply.cs

@@ -163,7 +163,13 @@ public class DefaultDieselPowerSupply : DieselPowerSupply
         private Timer PowerOnTimer;
         private Timer AuxPowerOnTimer;
 
+        /// <remarks>
+        /// Used for the corresponding first engine on/off sound triggers.
+        /// </remarks>
         private DieselEngineState PreviousFirstEngineState;
+        /// <remarks>
+        /// Used for the corresponding second engine on/off sound triggers.
+        /// </remarks>
         private DieselEngineState PreviousSecondEngineState;
 
         private bool QuickPowerOn = false;