Support multiple aria-details ids

As part of work on ARIA Annotations, the ARIA spec is being updated to allow aria-details to support multiple ids. See w3c/aria#1136
The plan has been approved by screen reader developers as well as the ARIA WG.
(If it's not in yet, it's because we are still working on the wording).

Hmm, I haven't found the idl for ariaDetailsElement in this repo -- does it exist?
It needs to change to ariaDetailsElements.