Fieldset should support Description

[aaronadamsCA]

I think the Fieldset component should support the Description component.

This fieldset from GOV.UK Frontend has aria-describedby="nationality-hint nationality-error":

But this similar fieldset from Catalyst does not have aria-describedby:

I think <Description> would make this a lot better.

We discovered this limitation when trying to add an error message for a combined input like this one:

By using markup like this:

<Fieldset>
  <Legend />
  <div>
    <Field>
      <Label />
      <Select />
    </Field>
    <Field>
      <Label />
      <Input />
    </Field>
  </div>
  <Description />
</Fieldset>

We hit a rendering error due to the missing description provider.

I think this pattern should be supported. I'd be happy to contribute the change if it's as straightforward as it looks. Thanks! 🙌