docs(di): add clarification for injection

docs/content/guide/di.ngdoc

@@ -284,8 +284,11 @@ Here is an example of using the injector service:
 var myModule = angular.module('myModule', []);
 ```
 
+Notice that `myModule` is independent as array is empty here.
+Dependent is always need dependency object to be injected inside it.
 Teach the injector how to build a `greeter` service. Notice that `greeter` is dependent on the
 `$window` service. The `greeter` service is an object that contains a `greet` method.
+so, injector now injects `$window` dependency inside `greeter` service.
 
 ```js
 myModule.factory('greeter', function($window) {