docs($injector): add example on how to use the element.injector

Closes angular#5188

Author: joscha

src/auto/injector.js

@@ -27,6 +27,28 @@
  *     $rootScope.$digest();
  *   });
  * </pre>
+ * 
+ * Sometimes you want to get access to the injector of a currently running Angular app
+ * from outside Angular. Perhaps, you want to inject and compile some markup after the
+ * application has been bootstrapped. You can do this using extra `injector()` added
+ * to JQuery/jqLite elements. See {@link angular.element}.
+ * 
+ * *This is fairly rare but could be the case if a third party library is injecting the
+ * markup.*
+ * 
+ * In the following example a new block of HTML containing a `ng-controller`
+ * directive is added to the end of the document body by JQuery. We then compile and link
+ * it into the current AngularJS scope.
+ * 
+ * <pre>
+ * var $div = $('<div ng-controller="MyCtrl">{{content.label}}</div>');
+ * $(document.body).append($div);
+ * 
+ * angular.element(document).injector().invoke(function($compile) {
+ *   var scope = angular.element($div).scope();
+ *   $compile($div)(scope);
+ * });
+ * </pre>
  */