add note about top level ng1 app

Author: filipesilva

public/docs/ts/latest/guide/upgrade.jade

@@ -413,7 +413,7 @@ figure.image-display
 
   The first step to upgrading an application using the `UpgradeModule` is
   always to bootstrap it as a hybrid that supports both Angular 1 and
-  Angular 2.
+  Angular 2, but still is an Angular 1 app at top level.
 
   Pure Angular 1 applications can be bootstrapped in two ways: By using an `ng-app`
   directive somewhere on the HTML page, or by calling