docs(ngController): do not attach a controller twice

A common mistake for beginners is to attach a controller in both the
$routeProvider and also in the html document using the ng-controller
directive. This change highlights this, to help prevent developers from
doing so in the future.

Closes #4409

Author: tylermcginnis

src/ng/directive/ngController.js

@@ -16,7 +16,10 @@
  * * Controller — The `ngController` directive specifies a Controller class; the class contains business
  *   logic behind the application to decorate the scope with functions and values
  *
- * Note that an alternative way to define controllers is via the {@link ngRoute.$route $route} service.
+ * Note that you can also attach controllers to the DOM by declaring it in a route definition
+ * via the {@link ngRoute.$route $route} service. A common mistake is to declare the controller
+ * again using `ng-controller` in the template itself.  This will cause the controller to be attached
+ * and executed twice.
  *
  * @element ANY
  * @scope