docs($ionicModal): clearly note that .modal wrapper is required

Closes #1384.

Just to avoid mistakes/questions like this http://forum.ionicframework.com/t/ionic-modal-not-animating/4196

Author: felquis

js/angular/controller/scrollController.js

@@ -165,8 +165,6 @@ function($scope, scrollViewOptions, $timeout, $window, $$scrollValueCache, $loca
     }
   };
 
-
-
   /**
    * @private
    */

js/angular/service/modal.js

@@ -4,7 +4,8 @@
  * @module ionic
  * @description
  * The Modal is a content pane that can go over the user's main view
- * temporarily.  Usually used for making a choice or editing an item.
+ * temporarily.  Usually used for making a choice or editing an item. 
+ * Note that you need to put the content of the modal inside a div with the class `modal`.
  *
  * @usage
  * ```html