[Editorial] Clarify what "disallowed" values are (w3c#7585)

This text is based on that in Cascade 4.

Author: gsnedders

mediaqueries-4/Overview.bs

@@ -1023,7 +1023,8 @@ Error Handling</h3>
 		rather than just treating the ''or'' as an unknown <a>media type</a>.
 	</div>
 
-	An unknown <<mf-name>> or <<mf-value>>, or disallowed <<mf-value>>,
+	An unknown <<mf-name>> or <<mf-value>>, or
+	a feature value which does not matches the value syntax for that <a>media feature</a>,
 	results in the value “unknown”.
 	A <<media-query>> whose value is “unknown” must be replaced with ''not all''.
 

mediaqueries-5/Overview.bs

@@ -1021,7 +1021,8 @@ Error Handling</h3>
 		rather than just treating the ''or'' as an unknown <a>media type</a>.
 	</div>
 
-	An unknown <<mf-name>> or <<mf-value>>, or disallowed <<mf-value>>,
+	An unknown <<mf-name>> or <<mf-value>>, or
+	a feature value which does not matches the value syntax for that <a>media feature</a>,
 	results in the value “unknown”.
 	A <<media-query>> whose value is “unknown” must be replaced with ''not all''.