[css-lists-3] Add note about lack of interop for undefined behavior.

Author: fantasai

css-lists-3/Overview.bs

@@ -1075,6 +1075,9 @@ Counters in elements that do not generate boxes</h3>
 	can set, reset, or increment a counter
 	is undefined.
 
+	Note: The behavior on [=replaced element=] descendants is currently undefined
+	due to a lack of interoperability across implementations.
+
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