[css-2020] Mention that rough interop specs are themselves not well specified.

Author: fantasai

css-2020/Overview.bs

@@ -291,7 +291,7 @@ Classification of CSS Specifications</h2>
 Modules with Rough Interoperability</h3>
 
 	The following modules are widely deployed with <a>rough interoperability</a>,
-	but the details are not fully worked out
+	but the details are not fully worked out or sufficiently well-specified
 	and they need more testing and bugfixing.
 	We hope to incorporate them into the [[#css|official definition of CSS]] in a future snapshot.