docs($document): add a documentation example.

The $document docs are pretty empty, and this fills them out a bit. The example itself may not be
particularly useful, but it could be improved or removed later. Works for me.

Closes #6757

src/ng/document.js

@@ -7,6 +7,22 @@
  *
  * @description
  * A {@link angular.element jQuery or jqLite} wrapper for the browser's `window.document` object.
+ *
+ * @example
+   <example>
+     <file name="index.html">
+       <div ng-controller="MainCtrl">
+         <p>$document title: <b ng-bind="title"></b></p>
+         <p>window.document title: <b ng-bind="windowTitle"></b></p>
+       </div>
+     </file>
+     <file name="script.js">
+       function MainCtrl($scope, $document) {
+         $scope.title = $document[0].title;
+         $scope.windowTitle = angular.element(window.document)[0].title;
+       }
+     </file>
+   </example>
  */
 function $DocumentProvider(){
   this.$get = ['$window', function(window){