[mediaqueries-5] Fix authoring advice so future UAs don't fall into unknown territory. w3c#3857 https://logs.csswg.org/irc.w3.org/css/2020-05-27/#e1319612 https://logs.csswg.org/irc.w3.org/css/2020-05-28/#e1319862

Author: fantasai

mediaqueries-5/Overview.bs

@@ -831,9 +831,8 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
 	(to express a more active preference for light color schemes,
 	or preferences for other types of color schemes like "sepia").
 	As such, the most future-friendly way to use this media feature
-	is by directly testing against the values you wish to match,
-	such as ''(prefers-color-scheme: light)'' and ''(prefers-color-scheme: dark)'',
-	rather than by negation such as ''(prefers-color-scheme: light)'' and ''(not (prefers-color-scheme: light))''.
+	is by negation such as ''(prefers-color-scheme: dark)'' and ''(not (prefers-color-scheme: dark))'',
+	which ensures that new values fall into at least one of the styling blocks.
 
 	The method by which the user expresses their preference can vary.
 	It might be a system-wide setting exposed by the Operating System,