Add some javadoc to update

lib/update.js

@@ -39,6 +39,10 @@ function updateArray(updates, obj) {
  * Or with a function:
  * update({ foo: x => (x + 1) }, { foo: 2 });
  * // => { foo: 3 }
+ *
+ * @param {Object|Function} updates
+ * @param {Object|Array}    object to update
+ * @return {Object|Array}   new object with modifications
  */
 function update(updates, obj) {
   if (typeof updates === 'function') {