[css-color-5] update changes to 27 Apr, for publication

Author: svgeesus

css-color-5/Overview.bs

@@ -1023,7 +1023,10 @@ Changes {#changes}
 
 <ul>
 
-	<!-- to 20 Apr 2021 -->
+	<!-- to 27 Apr 2021 -->
+	<li>Added examples of percentage normalization in color-mix()</li>
+	<li>Explicitly excluded negative percentages in color-mix()</li>
+	<li>Percentages in color-mix() summing to less than 100% produce an alpha transparency less than 100%</li>
 	<li>Consistently used the term color space rather than colorspace,
 		defined <<color-space>> token
 	</li>