docs(guide/injecting_controllers): add a hint in example

Add a hint to tell the user that they need to click 3 times before an alert is shown.

docs/content/guide/dev_guide.services.injecting_controllers.ngdoc

@@ -98,6 +98,7 @@ function myController($scope, notify) {
   <p>Let's try the notify service, that is implicitly injected into the controller...</p>
   <input ng-init="message='test'" ng-model="message">
   <button ng-click="callNotify(message);">NOTIFY</button>
+  <p>(you have to click 3 times to see an alert)</p>
 </div>
 </doc:source>
 </doc:example>