[mediaqueries-5] Clarify prefers-color-scheme.

Fixes #3278.

Author: grorg

mediaqueries-5/Overview.bs

@@ -49,7 +49,7 @@ Evaluating Media Features in a Boolean Context</h4>
 	<em>other than</em> the number ''0'',
 	a <<dimension>> with the value ''0'',
 	the keyword ''@media/update/none'',
-	or a value explicitely defined by that media feature to evaluate as false in a boolean context,
+	or a value explicitly defined by that media feature to evaluate as false in a boolean context,
 	the <a>media feature</a> evaluates to true.
 	Otherwise, it evaluates to false.
 
@@ -622,8 +622,8 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 	Type: discrete
 	</pre>
 
-	The 'prefers-color-scheme' media feature is used to detect if the user
-	has requested the page use a light or dark color theme.
+	The 'prefers-color-scheme' media feature reflects the user's
+	desire that the page use a light or dark color theme.
 
 	<dl dfn-type=value dfn-for="@media/prefers-color-scheme">
 		<dt><dfn>no-preference</dfn>
@@ -634,18 +634,22 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 
 		<dt><dfn>light</dfn>
 		<dd>
-			Indicates that user has notified the system that they
-			prefer a page that has a light theme (dark text on light background).
+			Indicates that user has expressed the preference for
+			a page that has a light theme (dark text on light background).
 
 		<dt><dfn>dark</dfn>
 		<dd>
-			Indicates that user has notified the system that they
-			prefer a page that has a dark theme (light text on dark background).
+			Indicates that user has expressed the preference for
+			a page that has a dark theme (light text on dark background).
 
 	</dl>
 
-	Note: User preferences can vary by medium;
-	for example, a user may prefer dark themes on a glowing screen,
+	The method by which the user expresses their preference can vary.
+	It might be a system-wide setting exposed by the Operating System,
+	or a setting controlled by the User Agent.
+
+	Note: User preferences can also vary by medium.
+	For example, a user may prefer dark themes on a glowing screen,
 	but light themes when printing
 	(to save ink
 	and/or because inked text on blank paper prints better