Clarify what 'unknown' does for things other than <media-query>. Clarify what unknown media features turn into in general.

Author: tabatkins

mediaqueries/Overview.bs

@@ -782,8 +782,15 @@ Syntax</h2>
 			The result is the result of evaluating the specified media feature.
 	</dl>
 
-	If the result of a <a>media query</a> is unknown,
-	it must be treated as false for the purpose of matching.
+	If the result of any of the above productions
+	is used in any context that expects a two-valued boolean,
+	"unknown" must be converted to "false".
+
+	Note: This means that,
+	for example,
+	when a <a>media query</a> is used in a ''@media'' rule,
+	if it resolves to "unknown" it's treated as "false"
+	and fails to match.
 
 	<div class="note">
 		Media Queries use a three-value logic where terms can be "true", "false", or "unknown".
@@ -800,7 +807,7 @@ Syntax</h2>
 		This logic was adopted because <<general-enclosed>> needs to be assigned a truth value.
 		In standard boolean logic, the only reasonable value is "false",
 		but this means that ''not unknown(function)'' is true,
-		which can be confusing an unwanted.
+		which can be confusing and unwanted.
 		Kleen's 3-valued logic ensures that unknown things will prevent a <a>media query</a> from matching,
 		unless their value is irrelevant to the final result.
 	</div>
@@ -856,7 +863,8 @@ Error Handling</h3>
 	</div>
 
 	An unknown <<mf-name>> or <<mf-value>>, or disallowed <<mf-value>>,
-	must make the entire <<media-query>> be replaced by ''not all''.
+	results in the value "unknown".
+	A <<media-query>> whose value is "unknown" must be replaced with ''not all''.
 
 	<div class="example">
 		<pre>&lt;link media="screen and (max-weight: 3kg) and (color), (color)"