[css-shadow-parts-1] Reference IDL issue

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -133,6 +133,8 @@ with keys that are [=strings=]
 and values that are [=ordered sets=] of [=strings=]
 (part names that are selectable inside this element).
 
+Issue(w3c/csswg-drafts#2414): Document how the part name map will be exposed via IDL.
+
 Each <a>shadow root</a> can be thought of as having a <dfn export for="shadow root">part element map</dfn>
 with keys that are [=strings=]
 and values that are [=ordered sets=] of elements.