Support objects for FormGroup legendText prop

[queeniema]

Summary

I am attempting to create a group of radio buttons using the example code in the storybook here.

<FormGroup legendText="My radio button group label">
  <RadioButtonGroup
    name="radio-button-group"
    onChange={onRadioButtonChange}
    orientation="vertical"
    valueSelected="radio-button-1"
  >
    <RadioButton
      id="radio-button-1"
      labelText="First radio button"
      value="radio-button-1"
    />
    <RadioButton
      id="radio-button-2"
      labelText="Second radio button"
      value="radio-button-2"
    />
    <RadioButton
      id="radio-button-3"
      labelText="Third radio button"
      value="radio-button-3"
    />
  </RadioButtonGroup>
</FormGroup>

I would like to use a Tooltip that contains help information for the legendText prop for the FormGroup component, like the following:

<FormGroup legendText={(
  <Tooltip
    triggerText="My radio button group label"
    direction="right"
  >
    Some help information here...
  </Tooltip>
)}>
// ... RadioButtonGroup here ...
</FormGroup>

However, it seems that only string values are supported. I see this error:

Warning: Failed prop type: Invalid prop `legendText` of type `object` supplied to `FormGroup`, expected `string`.

Justification

We should be able to display help information if a user needs it.

Also, this is supported for the labelText prop for the TextInput component, so I think it makes sense to have it supported for radio button groups as well.