[css-scrollbar] Beef-up note about scrollbar colors and interaction

See w3c#10591

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -289,8 +289,13 @@ Value Definitions</h3>
 	(for example, when users choose a configuration option/setting that
 	always ensures a particular scrollbar color / use of system default scrollbars).
 
-	Note: When a user interacts with a scrollbar (e.g. hovering or activating),
-	implementations can alter which scrollbar colors apply to which scrollbar parts.
+	Note: Implementations can take advantage of the fact that they have full discretion
+	in how to apply the specified colors
+	in order to visually respond to user interactions with the scrollbar
+	(e.g. hovering or activating):
+	they could alter which scrollbar parts the specified colors apply to,
+	using darker or lighter shades of the specified colors,
+	more or less saturated variants…
 
 	If the user agent displays the scrollbar without the [=track=],
 	they must ensure reasonable contrast between the [=thumb=] and what is behind it.