docs(toast): document string argument for #showSimple

closes #2930

src/components/toast/toast.js

@@ -61,16 +61,17 @@ function MdToastDirective($mdToast) {
   */
 
 /**
-  * @ngdoc method
-  * @name $mdToast#showSimple
-  *
-  * @description
-  * Convenience method which builds and shows a simple toast.
-  *
-  * @returns {promise} A promise that can be resolved with `$mdToast.hide()` or
-  * rejected with `$mdToast.cancel()`.
-  *
-  */
+ * @ngdoc method
+ * @name $mdToast#showSimple
+ * 
+ * @param {string} message The message to display inside the toast
+ * @description
+ * Convenience method which builds and shows a simple toast.
+ *
+ * @returns {promise} A promise that can be resolved with `$mdToast.hide()` or
+ * rejected with `$mdToast.cancel()`.
+ *
+ */
 
  /**
   * @ngdoc method