[css-shadow-parts-1] Add more detail on IDL attributes

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -368,11 +368,15 @@ but never match additional <a>shadow-part pseudo-elements</a>.
 Extensions to the {{Element}} Interface {#idl}
 ==============================================
 
-We expose the `part` and `exposedparts` attributes as read/write string properties on Element.
-
 <pre class=idl>
 partial interface Element {
   attribute DOMString part;
   attribute DOMString exposedParts;
 };
 </pre>
+
+The `exposedParts` attribute must reflect the `exposedparts` content attribute.
+
+The `part` attribute must reflect the `part` content attribute.
+
+Issue(w3c/csswg-drafts#2414): Define these as superglobals in the DOM spec.