Provide caveats to when role=none may be allowed

[scottaohara]

The typical rule for button and input elements is that role=none | presentation is not allowed. This makes sense because the conflict resolution for the role would result in it immediately being ignored due to these elements being focusable.

However, if one were to also specify the disabled attribute on these elements, then the element is no longer focusable and so long as there are not any other conflicts with the role, it will make these elements return as 'presentational'.

e.g., <button disabled role=none>yo</button>

This issue should be considered in tandem with #365