Avoid XML-specific terminology "IDREF"

https://w3c.github.io/aria/ defines "ID reference" and "ID reference list", but also "Valid IDREF" (not used?), and goes on to use "IDREF" and "IDREFS" in places where it should say "ID reference" or "ID reference list", as far as I can tell.

Since IDREF and IDREFS are XML-specific terminology, these should be avoided (except when specifically talking about those concepts).