docs(Angular.js): add missing @returns to extend()

src/Angular.js

@@ -238,6 +238,7 @@ function setHashKey(obj, h) {
  *
  * @param {Object} dst Destination object.
  * @param {...Object} src Source object(s).
+ * @returns {Object} Reference to `dst`.
  */
 function extend(dst) {
   var h = dst.$$hashKey;