[css-conditional-3] update changes since CR Snapshot

Author: svgeesus

css-conditional-3/Overview.bs

@@ -710,10 +710,20 @@ This information can also, in aggregate, be used to improve the accuracy of
 <h2 id="changes">
 Changes</h2>
 
+The following (non-editorial) changes were made to this specification since the
+<a href="https://www.w3.org/TR/2020/CR-css-conditional-3-20201208/">8 December 2020 Candidate Recommendation Snapshot</a>:
+
+<ul id="changes-20201208">
+  <!-- to 6 Oct 2021 -->
+  <li>Defined feature queries ("supports" queries)</li>
+  <li>Removed the "unknown" value</li>
+  <li>Clarified placement rules</li>
+</ul>
+
 The following (non-editorial) changes were made to this specification since the
 <a href="https://www.w3.org/TR/2013/CR-css3-conditional-20130404/">4 April 2013 Candidate Recommendation</a>:
 
-<ul>
+<ul id="changes-20130404">
   <!-- to 1 Dec 2020 -->
   <li>Clarified that namespaces in conditionText are invalid</li>
   <li>New editors added</li>