[css-ui-4] Additional guidance against abuse of user-select none.

Author: frivoal

css-ui-4/Overview.bs

@@ -540,6 +540,11 @@ Omitted values are set to their initial values.
 		make it easier for the user to select the content they want,
 		by letting the author disable selection on UI elements
 		that are not useful to select.
+		Tools such as CSS validators, linters or in-browser developer tools
+		are encouraged to use heuristics
+		to detect and warn against incorrect or abusive usage
+		that would hamper usability
+		or violate common user expectations.
 
 		<dt><dfn>element</dfn>
 		<dd>UAs must not allow a selection which is started in this element