docs(checkbox): add storybook docs for onClick

packages/react/src/components/Checkbox/Checkbox.mdx

@@ -91,6 +91,23 @@ event.
 <Checkbox onChange={()=>{}} labelText={`Checkbox label`} id="checkbox-label-2" />
 ```
 
+### Checkbox onClick
+
+You can use the `onClick` prop to pass in a custom function for event handling.
+This event can be used to control click event bubbling without having to
+implement your own wrapping element/component. It receives the dom event as its
+only argument.
+
+`onClick` is placed on the wrapping `div` containing both the `input` and
+`label`. As such, this will be called twice for every click - once for the
+input, a second time for the label. Due to this, `onClick` should not be used to
+catch state updates when using Checkbox as a controlled component.
+
+```jsx
+<Checkbox onClick={()=>{}} labelText={`Checkbox label`} id="checkbox-label-1" />
+<Checkbox onClick={()=>{}} labelText={`Checkbox label`} id="checkbox-label-2" />
+```
+
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