[mediaqueries-5] Change dynamic-range to nested sets rather than alternatives

Closes #6793

Author: frivoal

mediaqueries-5/Overview.bs

@@ -1889,22 +1889,33 @@ Dynamic Range: the 'dynamic-range' feature</h3>
 	</pre>
 
 '@media/dynamic-range' represents the combination of max brightness,
-color depth, and contrast ratio that are supported by the UA and output device.
+color depth, and contrast ratio that are supported by the user agent and output device.
 
 <dl dfn-type=value dfn-for="@media/dynamic-range">
 	<dt><dfn>high</dfn>
 	<dd>
-		The combination of the user agent and the output device
+		The user agent and the output device
 		fulfill all of the following criteria:
 		* it has a [=high peak brightness=]
 		* it has a [=high contrast ratio=]
 		* its color depth is greater than 24 bit or 8 bit per color component of RGB
+
+		Note: Some devices have high dynamic range capabilities that are not always on,
+		and that need to be activated
+		(sometimes programmatically, sometimes by the user, sometimes based on the content).
+		This media feature does not test whether such a mode is active,
+		just whether the device is capable of high dynamic range visuals.
+
 	<dt><dfn>standard</dfn>
 	<dd>
-		One or more of the criteria for a ''dynamic-range/high'' 'dynamic-range'
-		is not fulfilled.
+		This value matches on any visual device,
+		and not on devices lacking visual capabilities.
 </dl>
 
+	Note: More than one value of this media feature can match simulataneously:
+	a user agent matching ''dynamic-range/high''
+	will also match ''dynamic-range/standard''.
+
 <h4 id="contrast-brightness-of-display">
 	Determining contrast and brightness of display</h4>