[ZEPPELIN-4280] [Followup] Add comments

### What is this PR for?
This is followup issue for ZEPPELIN-4280, to add some comments on the code.

### What type of PR is it?
Documentation

### What is the Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-4280

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: Lee moon soo <moon@apache.org>

Closes apache#3461 from Leemoonsoo/ZEPPELIN-4280-followup and squashes the following commits:

75d4995 [Lee moon soo] add comment

(cherry picked from commit 7d74b92)
Signed-off-by: Lee moon soo <moon@apache.org>

zeppelin-web/src/app/notebook/paragraph/result/result.controller.js

@@ -802,9 +802,13 @@ function ResultCtrl($scope, $rootScope, $route, $window, $routeParams, $location
         newConfig.graph.values = newConfig.graph.commonSetting.pivot.values;
         delete newConfig.graph.commonSetting.pivot;
       }
+
+      // don't send commitParagraphResult when config is the same.
+      // see https://issues.apache.org/jira/browse/ZEPPELIN-4280.
       if (angular.equals($scope.config, newConfig)) {
         return;
       }
+
       console.debug('committVizConfig', newConfig);
       let newParams = angular.copy(paragraph.settings.params);
       commitParagraphResult(paragraph.title, paragraph.text, newConfig, newParams);