docs(angular.copy): add an example with the two possible arguments

Closes #4179

src/Angular.js

@@ -589,6 +589,42 @@ function isLeafNode (node) {
  * @param {(Object|Array)=} destination Destination into which the source is copied. If
  *     provided, must be of the same type as `source`.
  * @returns {*} The copy or updated `destination`, if `destination` was specified.
+ *
+ * @example
+ <doc:example>
+ <doc:source>
+ <div ng-controller="Controller">
+ <form novalidate class="simple-form">
+ Name: <input type="text" ng-model="user.name" /><br />
+ E-mail: <input type="email" ng-model="user.email" /><br />
+ Gender: <input type="radio" ng-model="user.gender" value="male" />male
+ <input type="radio" ng-model="user.gender" value="female" />female<br />
+ <button ng-click="reset()">RESET</button>
+ <button ng-click="update(user)">SAVE</button>
+ </form>
+ <pre>form = {{user | json}}</pre>
+ <pre>master = {{master | json}}</pre>
+ </div>
+
+ <script>
+ function Controller($scope) {
+    $scope.master= {};
+
+    $scope.update = function(user) {
+      // Example with 1 argument
+      $scope.master= angular.copy(user);
+    };
+
+    $scope.reset = function() {
+      // Example with 2 arguments
+      angular.copy($scope.master, $scope.user);
+    };
+
+    $scope.reset();
+  }
+ </script>
+ </doc:source>
+ </doc:example>
  */
 function copy(source, destination){
   if (isWindow(source) || isScope(source)) throw Error("Can't copy Window or Scope");