[css-values-4] Add a note explaining fragment-only URL resolution a little better.

Author: tabatkins

css-values-4/Overview.bs

@@ -962,6 +962,12 @@ Fragment URLs</h5>
 	This reference must always be treated as same-document
 	(rather than cross-document).
 
+	Note: This means that stylesheets inside a [=shadow tree=]
+	will resolve these URLs only to elements inside the shadow,
+	elements in the light DOM
+	will resolve these URLs only to elements in the light DOM,
+	etc.
+
 	When <a href="https://www.w3.org/TR/cssom-1/#serializing-css-values">serializing</a>
 	a ''url()'' with the <a>local url flag</a> set,
 	it must serialize as just the fragment.