docs(select): document the multiple attribute

Add the `multiple` attribute to the documentation of the select directive.

Closes #13119

Author: Puigcerber

src/ng/directive/select.js

@@ -187,6 +187,8 @@ var SelectController =
  *
  * @param {string} ngModel Assignable angular expression to data-bind to.
  * @param {string=} name Property name of the form under which the control is published.
+ * @param {string=} multiple Allows multiple options to be selected. The selected values will be
+ *     bound to the model as an array.
  * @param {string=} required Sets `required` validation error key if the value is not entered.
  * @param {string=} ngRequired Adds required attribute and required validation constraint to
  * the element when the ngRequired expression evaluates to true. Use ngRequired instead of required