[mediaqueries-5] Clarify intent of prefers-reduced-motion

See w3c#5594

Author: frivoal

mediaqueries-5/Overview.bs

@@ -2771,8 +2771,8 @@ Detecting the desire for less motion on the page: the 'prefers-reduced-motion' f
 	</pre>
 
 	The 'prefers-reduced-motion' media feature is used to detect if the user
-	has requested the system minimize the amount of animation or motion it
-	uses.
+	has requested the system minimize the amount of non-essential motion
+	it uses.
 
 	<dl dfn-type=value dfn-for="@media/prefers-reduced-motion">
 		<dt><dfn>no-preference</dfn>
@@ -2783,11 +2783,11 @@ Detecting the desire for less motion on the page: the 'prefers-reduced-motion' f
 
 		<dt><dfn>reduce</dfn>
 		<dd>
-			Indicates that user has notified the system that they
-			prefer an interface that minimizes the amount of movement
-			or animation, preferably to the point where all non-essential
-			movement is removed.
-
+			Indicates that user has notified the system
+			that they prefer an interface that
+			removes or replaces the types of motion-based animation
+			that either trigger discomfort for those with vestibular motion sensitivity,
+			or distraction for those with attention deficits.
 	</dl>
 
 <h3 id="prefers-reduced-transparency">