[css-ui-4] Add note about copyability

Author: frivoal

css-ui-4/Overview.bs

@@ -708,6 +708,12 @@ Omitted values are set to their initial values.
 		and they may include this element.
 		If the element has descendants on which 'user-select' does not compute to ''user-select/none'',
 		these descendants must be included in a selection extending across the element.
+		<span class=note>This specification makes no normative requirement
+		about the behavior of the clipboard.
+		however, UAs are encouraged to keep the visual selection consistent
+		with what would get copied to the clipboard when copying.
+		Copying text that does not appear to be selected, or vice versa,
+		is highligh confusing to users.</span>
 
 		As 'user-select' is a UI convenience mechanism,
 		not a copy protection mechanism,