[css-view-transitions-2] view-transition-name: auto with ID should only match other IDs (w3c#11930)

* [css-view-transitions-2] `view-transition-name: auto` with ID should only match other IDs

Closes w3c#11614

* Update css-view-transitions-2/Overview.bs

Co-authored-by: vmpstr <vmpstr@chromium.org>

* Update Overview.bs

---------

Co-authored-by: vmpstr <vmpstr@chromium.org>

Author: noamr

css-view-transitions-2/Overview.bs

@@ -815,7 +815,8 @@ To resolve the [=used value=] of 'view-transition-name' for |element|:
 	1. If |computed| is <css>none</css>, return null.
 	1. If |computed| is a <<custom-ident>>, return |computed|.
 	1. Assert: |computed| is <css>auto</css> or <css>match-element</css>.
-	1. If |computed| is <css>auto</css>, |element| has an associated [=Element/id=], and |computed| is associated with the same [=tree/root=] as |element|'s [=tree/root=], then return the value of |element|'s [=Element/id=].
+	1. If |computed| is <css>auto</css>, |element| has an associated [=Element/id=], and |computed| is associated with the same [=tree/root=] as |element|'s [=tree/root=], then return a unique string.
+		Two elements with the same [=Element/id=] must return the same string, regardless of their [=node document=].
 
 		Note: this means that a ''::part()'' pseudo-element wouldn't resolve to its matching element's [=Element/id=].