add explanation for len attribute in handleHistoryChange

lib/src/widgets/quill/quill_controller.dart

@@ -245,6 +245,8 @@ class QuillController extends ChangeNotifier {
   }
 
   void _handleHistoryChange(int? len) {
+    // move cursor according to the length inserted or deleted from redo or undo
+    // operation. len is the length inserted or deleted.
     if (len! != 0) {
       // if (this.selection.extentOffset >= document.length) {
       // // cursor exceeds the length of document, position it in the end