fix(ui-checkbox): warn CheckboxGroup onChange

[buckett]

CheckboxGroup overrides the onChange value of contained Checkbox components. If a developer incorrectly sets an onChange property on the Checkbox contained within a CheckboxGroup we write a warning to the console in the standard way.

The behaviour is also added to the documentation in the "Guidelines -> Don't" section.

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[matyasf]

Thanks for the contribution! We are looking into it

[matyasf]

Can you show a code snippet where "CheckboxGroup overrides the onChange value of contained Checkbox components"? I've got this code:

<FormFieldGroup description={<ScreenReaderContent>CheckboxGroup examples</ScreenReaderContent>}>
  <CheckboxGroup name="sports"
    onChange={function (value) { console.log('group onchange ' + value) }}
    defaultValue={['football', 'volleyball']}
    description="Select your favorite sports"
  >
    <Checkbox label="Football" value="football" onChange={function (value) { console.log('Football CB onchange ' + value) }}/>
    <Checkbox label="Basketball" value="basketball" />
    <Checkbox label="Volleyball" value="volleyball" />
    <Checkbox label="Other" value="other" />
  </CheckboxGroup>
</FormFieldGroup>

and both the group and the individual checkbox event listeners are triggered. In the code I see that they should work, the group clones the checkboxes and safeCloneElement should keep both listeners https://github.com/instructure/instructure-ui/blob/master/packages/ui-react-utils/src/safeCloneElement.ts#L97