Add docs for HelpText to CheckboxGroup and RadioGroup (#3528)
* add to checkbox group
* add to radio group
* add to CheckboxGroup docs
* add to RadioGroup docs
* add to useCheckboxGroup docs
* add to useRadioGroup docs
* update error display logic in docs
* fix merge conflict
* fix merge
* example update
Co-authored-by: Danni <drobinson@livefyre.com>