[mediaqueries] Clarify boolean context to make it clearer that MQs are false in non-applicable contexts.

Author: tabatkins

mediaqueries/Overview.html

@@ -774,8 +774,13 @@ <h4 class="heading settled heading" data-level=2.4.2 id=mq-boolean-context><span
 	If the feature would be true for the number <span class=css data-link-type=maybe title=0>0</span>,
 	a dimension with the value <span class=css data-link-type=maybe title=0>0</span>,
 	or the keyword <a class=css data-link-type=maybe href=#valuedef-none0 title=none>none</a>,
-	the <a data-link-type=dfn href=#media-feature title="media feature">media feature</a> is false.
-	Otherwise, it’s true.
+	the <a data-link-type=dfn href=#media-feature title="media feature">media feature</a> evaluates to false.
+	If it would be true for any values <strong>other than</strong> the above,
+	it evaluates to true.
+	Otherwise, it evaluates to false.
+
+<p class=note>	Note: The “trichotomy” above allows for correct handling of MQs as false when on devices where they don’t apply at all,
+	such as <a class=property data-link-type=propdesc href=#descdef-scan title=scan>scan</a> on a speech device.
 
 	<div class=example>
 		Some <a data-link-type=dfn href=#media-feature title="media features">media features</a> are designed to be written like this,
@@ -803,7 +808,9 @@ <h4 class="heading settled heading" data-level=2.4.2 id=mq-boolean-context><span
 
 <p>		For example, <span class=css data-link-type=maybe title=(pointer)>(pointer)</span> is useful,
 		as <a class=property data-link-type=propdesc href=#descdef-pointer title=pointer>pointer</a> has a <a class=css data-link-for=pointer data-link-type=maybe href=#valuedef-none0 title=none>none</a> value to indicate there’s no pointing device at all on the device.
-		On the other hand, <span class=css data-link-type=maybe title=(scan)>(scan)</span> is just always true, as there’s no value that means “false”.
+		On the other hand, <span class=css data-link-type=maybe title=(scan)>(scan)</span> is just always true or always false
+		(depending on whether it applies at all to the device),
+		as there’s no value that means “false”.
 	</div>
 
 

mediaqueries/Overview.src.html

@@ -471,8 +471,13 @@ <h4 id='mq-boolean-context'>
 	If the feature would be true for the number ''0'',
 	a dimension with the value ''0'',
 	or the keyword ''none'',
-	the <a>media feature</a> is false.
-	Otherwise, it's true.
+	the <a>media feature</a> evaluates to false.
+	If it would be true for any values <strong>other than</strong> the above,
+	it evaluates to true.
+	Otherwise, it evaluates to false.
+
+	Note: The “trichotomy” above allows for correct handling of MQs as false when on devices where they don't apply at all,
+	such as 'scan' on a speech device.
 
 	<div class='example'>
 		Some <a>media features</a> are designed to be written like this,
@@ -500,7 +505,9 @@ <h4 id='mq-boolean-context'>
 
 		For example, ''(pointer)'' is useful,
 		as 'pointer' has a ''pointer/none'' value to indicate there's no pointing device at all on the device.
-		On the other hand, ''(scan)'' is just always true, as there's no value that means “false”.
+		On the other hand, ''(scan)'' is just always true or always false
+		(depending on whether it applies at all to the device),
+		as there's no value that means “false”.
 	</div>