docs(radioButton): ngModel and ngChange are mdRadioGroup properties (#10473)

`ngModel` and `ngChange` are part of the `mdRadioGroup` directive and  not of the `mdRadioButton` directive.

Thx to @sebakerckhof!
Fixes #10421.

Author: ThomasBurleson

src/components/radioButton/radio-button.js

@@ -32,6 +32,9 @@ angular.module('material.components.radioButton', [
  *
  * @param {string} ng-model Assignable angular expression to data-bind to.
  * @param {boolean=} md-no-ink Use of attribute indicates flag to disable ink ripple effects.
+ * @param {string} ngModel Assignable angular expression to data-bind to.
+ * @param {string=} ngChange AngularJS expression to be executed when input changes due to user
+ *    interaction with the input element.
  *
  * @usage
  * <hljs lang="html">
@@ -224,9 +227,6 @@ function mdRadioGroupDirective($mdUtil, $mdConstant, $mdTheming, $timeout) {
  * the `<md-radio-button>` directive provides ink effects, ARIA support, and
  * supports use within named radio groups.
  *
- * @param {string} ngModel Assignable angular expression to data-bind to.
- * @param {string=} ngChange AngularJS expression to be executed when input changes due to user
- *    interaction with the input element.
  * @param {string} ngValue AngularJS expression which sets the value to which the expression should
  *    be set when selected.
  * @param {string} value The value to which the expression should be set when selected.
@@ -241,7 +241,7 @@ function mdRadioGroupDirective($mdUtil, $mdConstant, $mdTheming, $timeout) {
  *   Label 1
  * </md-radio-button>
  *
- * <md-radio-button ng-model="color" ng-value="specialValue" aria-label="Green">
+ * <md-radio-button ng-value="specialValue" aria-label="Green">
  *   Green
  * </md-radio-button>
  *