[css-anchor-position] Mention anchor-scope explicitly in the definition of 'acceptable anchor element'.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -384,6 +384,10 @@ or nothing (a missing specifier).
 		or a [=part-like pseudo-element=].
 
 	* |el| is not in the [=skipped contents=] of another element.
+
+	* |el| is in scope for |query el|,
+		per the effects of 'anchor-scope' on |query el|
+		or its ancestors.
 </div>
 
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