docs: update angular.extend() documentation

How to preserve original objects similar to jQuery.extend().

Closes #9087

src/Angular.js

@@ -334,6 +334,9 @@ function setHashKey(obj, h) {
  * Extends the destination object `dst` by copying own enumerable properties from the `src` object(s)
  * to `dst`. You can specify multiple `src` objects.
  *
+ * You can specify multiple `src` objects. If you want to preserve original objects, you can do so
+ * by passing an empty object as the target: `var object = angular.extend({}, object1, object2)`.
+ *
  * @param {Object} dst Destination object.
  * @param {...Object} src Source object(s).
  * @returns {Object} Reference to `dst`.