[css-anchor-1] Fix the note to be more accurate, given the fix to tree-scoping. w3c#7916

Author: tabatkins

css-anchor-1/Overview.bs

@@ -354,11 +354,16 @@ might not be capable of anchoring the positioned element.
 	ensuring there is no possibly circularity
 	in anchor positioning.
 
-	Note: Anchor names are scoped to the tree the anchor element is in;
-	if the querying element is in a different [=shadow tree=],
-	they won't find each other.
-	[=Implicit anchor elements=] aren't necessarily limited in the same way,
-	but that might vary depending on the feature assigning the implicit anchor element.
+	Note: An 'anchor-name' defined by styles in one [=shadow tree=]
+	won't be seen by [=anchor functions=] in styles in a different [=shadow tree=],
+	preserving encapsulation.
+	However, <em>elements</em> in different [=shadow trees=]
+	can still anchor to each other,
+	so long as both the 'anchor-name' and [=anchor function=]
+	come from styles in the same tree,
+	such as by using ''::part()'' to style an element inside a shadow.
+	([=Implicit anchor elements=] also aren't intrinsically limited to a single tree,
+	but the details of that will depend on the API assigning them.)
 </div>
 
 An element can also have an <dfn export>implicit anchor element</dfn>,