Added docs to Cancel method

CoffeeVendingMachines/CoffeeVendingMachine.cs

@@ -43,6 +43,10 @@ public class CoffeeVendingMachine
         // no need to implement anything here
     }
 
+    /// <summary>
+    /// Cancels the current input.
+    /// </summary>
+    /// <returns>The withdrawn coins.</returns>
     public CoinDepot[] Cancel()
     {
         int count = 0;