docs($filter): add runnable example

Closes angular#6871

Author: jvans1

src/ng/filter.js

@@ -73,7 +73,24 @@
  *
  * @param {String} name Name of the filter function to retrieve
  * @return {Function} the filter function
- */
+ * @example
+   <example name="$filter" module="filterExample">
+     <file name="index.html">
+       <div ng-controller="MainCtrl">
+        <h3>{{ originalText }}</h3>
+        <h3>{{ filteredText }}</h3>
+       </div>
+     </file>
+
+     <file name="script.js">
+      angular.module('filterExample', [])
+      .controller('MainCtrl', function($scope, $filter) {
+        $scope.originalText = 'hello';
+        $scope.filteredText = $filter('uppercase')($scope.originalText);
+      });
+     </file>
+   </example>
+  */
 $FilterProvider.$inject = ['$provide'];
 function $FilterProvider($provide) {
   var suffix = 'Filter';