[css-display-3] Update Changes list

Author: fantasai

css-display-3/Overview.bs

@@ -1866,6 +1866,10 @@ Changes</h2>
 			Pulled in the 'order' property definition from [[CSS-FLEXBOX-1]],
 			since it also applies to [=grid items=].
 			(<a href="https://github.com/w3c/csswg-drafts/issues/5865">Issue 5865</a>)
+		<li id="change-import-visibility">
+			Imported the 'visibility' property definition from [[CSS2]]
+			and updated it to be more thorough and complete.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/6123">Issue 6123</a>)
 		<li id="change-internal-replaced">
 			Defined that [=replaced elements=] with a [=layout-internal=] 'display'
 			are treated as ''display: inline''.