[css-cascade-5] Document change to layer name requirements

Author: mirisuzanne

css-cascade-5/Overview.bs

@@ -1840,6 +1840,8 @@ Changes</h2>
 	* Switched the ordering of unlayered styles
 		from highest to lowest priority in the normal origins.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/6284">Issue 6284</a>)
+	* Reserved the CSS-wide keywords for future use in layer-names.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/6323">Issue 6323</a>)
 
 <h3 id="additions-l4">
 Additions Since Level 4</h3>