[css-scrollbars] Update changelog

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -302,6 +302,13 @@ from
 
 This appendix is <em>non-normative</em>.
 
+<h3 class="no-num" id="changes-since-2021-12-06">
+  Changes since the <a href="https://www.w3.org/TR/2021/CR-css-scrollbars-1-20211206/">6 December 2021 Candidate Recommendation</a>
+</h3>
+
+	* Switched the Animation type of 'scrollbar-width' to [=discrete=],
+		reflecting the fact that this property only takes keyword values.
+
 <h3 class="no-num" id="changes-since-2021-12-02">
   Changes since the <a href="https://www.w3.org/TR/css-scrollbars-1/">2021-12-02 Working Draft</a>
 </h3>