[css-box-3] Update Changes list

Author: fantasai

css-box-3/Overview.bs

@@ -465,6 +465,7 @@ Changes Since CSS Level 2 {#changes}
 	since <a href="https://www.w3.org/TR/CSS2/box.html">CSS Level 2</a>:
 	<ul>
 		<li>Adapting the prose slightly to account for vertical <a>writing modes</a>.
+		<li>Cross-linking to relevant concepts in [[css-break-3]] and [[css-backgrounds-3]].
 		<li>Defining the non-applicability of 'margin', 'padding' and their longhands
 			to [=ruby base containers=] and [=ruby annotation containers=] (at risk).
 		<li>Providing a [[#keywords|centralized common definition]]