Update radio-group docs

[hawkticehurst]

Feature request

Based on #390, update docs to clarify that the vscode-radio-group component is only designed to accept vscode-radio components but not vscode-checkbox's.

Be sure to include a note about:

The implementation of vscode-radio-group follows the HTML spec for the radiogroup role where:

A radiogroup is a type of select list that can only have a single entry, or radio, checked at any one time.

So having checkboxes that can all be selected inside a radio group breaks the specification and will result in errors.