[css-values-3] Tweak Changes list

Author: fantasai

css-values-3/Overview.bs

@@ -2235,7 +2235,7 @@ Changes</h2>
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7287">Issue 7287</a>)
 	* Cleaned up interaction of this specification and [[CSS-CASCADE-3]] in defining the [=CSS-wide keywords=].
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7439">Issue 7439</a>)
-	* Removed definition of <<length-number>>.
+	* Removed definition of <<length-number>>, since it is impossible to combine them with ''calc()''.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/2789">Issue 2789</a>)
 	* Editorial improvements.