add comment for barcode reader cancel on web

@@ -148,7 +148,8 @@ class MobileScannerWeb extends MobileScannerPlatform {
       return;
     }
 
-    _barcodesSubscription?.cancel();
+    // Ensure the barcode scanner is stopped, by cancelling the subscription.
+    await _barcodesSubscription?.cancel();
     _barcodesSubscription = null;
 
     await _barcodeReader.stop();

@@ -160,6 +160,9 @@ final class ZXingBarcodeReader extends BarcodeReader {
       );
     };
 
+    // The onCancel() method of the controller is called
+    // when the stream subscription returned by this method is cancelled in `MobileScannerWeb.stop()`.
+    // This avoids both leaving the barcode scanner running and a memory leak for the stream subscription.
     controller.onCancel = () async {
       _reader?.stopContinuousDecode.callAsFunction();
       _reader?.reset.callAsFunction();