[css-2015] Clarify unstable.

Author: fantasai

css-2015/Overview.bs

@@ -472,10 +472,10 @@ Market Pressure and De Facto Standards</h4>
 
 	Issue: Added "should not" against encumbered features, per dbaron's request.
 
-	If a feature is <a>unstable</a>, yet
+	If a feature is <a>unstable</a> (i.e. the spec has not stabilized yet), yet
 
 	*	at least three UAs implement the feature
-		(or a UA has broken the other rules and shipped for broad use
+		(<em>or</em> a UA has broken the other rules and shipped for broad use
 		an <a>unstable</a> or otherwise non-standard feature in a production release),
 	*	<em>and</em> the implementations have rough interoperability,
 	*	<em>and</em> the CSS Working Group has recorded consensus