docs(Checkbox): disable checkbox group heading with knob (#7134)

Co-authored-by: Alessandra Davila <[email protected]>
Co-authored-by: TJ Egan <[email protected]>

packages/react/src/components/Checkbox/Checkbox-story.js

@@ -6,6 +6,7 @@
  */
 
 import React from 'react';
+import classNames from 'classnames';
 import { action } from '@storybook/addon-actions';
 import { withKnobs, boolean, text } from '@storybook/addon-knobs';
 import Checkbox from '../Checkbox';
@@ -63,7 +64,12 @@ const props = () => ({
 
 export const playground = () => (
   <fieldset className={`${prefix}--fieldset`}>
-    <legend className={`${prefix}--label`}>Checkbox heading</legend>
+    <legend
+      className={classNames(`${prefix}--label`, {
+        [`${prefix}--label--disabled`]: props().disabled,
+      })}>
+      Checkbox heading
+    </legend>
     <Checkbox {...props()} id="checkbox-label-1" />
     <Checkbox {...props()} id="checkbox-label-2" />
   </fieldset>