[mediaqueries] Allow literal true and false as custom MQ values, since you can hack always-true and always-false via boolean algebra already.

Author: tabatkins

mediaqueries/Overview.bs

@@ -1837,15 +1837,19 @@ Custom Media Queries</h2>
 
 	A <dfn>custom media query</dfn> is defined with the ''@custom-media'' rule:
 
-	<pre class='prod'><dfn>@custom-media</dfn> = @custom-media <<extension-name>> <<media-query-list>> ;</pre>
-
-	This defines that the <a>custom media query</a> named by the <<extension-name>> represents the given <<media-query-list>>.
+	<pre class='prod'>
+		<dfn>@custom-media</dfn> = @custom-media <<extension-name>> [ <<media-query-list>> | true | false ] ;
+	</pre>
 
 	The <<extension-name>> can then be used in a <a>media feature</a>.
 	It <strong>must</strong> be used in a <a>boolean context</a>;
 	using them in a normal or <a>range context</a> is a syntax error.
-	The <a>custom media query</a> evaluates to true if the <<media-query-list>> it represents evaluates to true,
+	If a <<media-query-list>> is given,
+	the <a>custom media query</a> evaluates to true
+	if the <<media-query-list>> it represents evaluates to true,
 	and false otherwise.
+	If <dfn value for="@custom-media">true</dfn> or <dfn value for="@custom-media">false</dfn> is given,
+	the <a>custom media query</a> evaluates to true or false, respectively.
 
 	<div class='example'>
 		For example, if a responsive site uses a particular breakpoint in several places,