Fixes a bug where <Modal> wasn't forwarding the `aria.labelledby` prop to
<ModalFrame> if the `title` prop was missing. However the purpose of
`labelledby` is to allow modals to provide their own title element, in
which case they won't want to use the `title` prop.

The `aria.labelledby` prop is now correctly forwarded to <ModalFrame> if
no title is provided. The `title` prop will take precedent over
`aria.labelledby`.

Make it clearer that while a title is required, it can be provided be
either the `title`, `aria.labelledby` or `contentLabel` props.