docs(bootstrap): Note that ngScenario requires ngApp

ngScenario expects an ngApp directive to be used, and doesn't work for
manually bootstrapped apps. The failure mode is to hang on navigation.

Trying to make this wont-fix bug less obscure by documenting it.
Eventually Protractor will replace ngScenario and fix this.

docs/content/guide/dev_guide.e2e-testing.ngdoc

@@ -302,3 +302,7 @@ element('.btn-danger').click();
     element('table tbody a');
     element('.btn-danger').click();
 </pre>
+
+# Caveats
+
+ngScenario does not work with apps that manually bootstrap using angular.bootstrap. You must use the ng-app directive.

src/Angular.js

@@ -999,6 +999,9 @@ function angularInit(element, bootstrap) {
  *
  * See: {@link guide/bootstrap Bootstrap}
  *
+ * Note that ngScenario-based end-to-end tests cannot use this function to bootstrap manually.
+ * They must use {@link api/ng.directive:ngApp ngApp}.
+ *
  * @param {Element} element DOM element which is the root of angular application.
  * @param {Array<String|Function>=} modules an array of module declarations. See: {@link angular.module modules}
  * @returns {AUTO.$injector} Returns the newly created injector for this app.