[css-multicol-4] Note issue about the different with CSS Figures

Author: frivoal

css-multicol-4/Overview.bs

@@ -165,6 +165,8 @@ Spanning columns</h2>
 				<li>How does that affect height calculations, and interract with column-fill
 			</ul>
 			</div>
+
+			Issue: <a href="https://figures.spec.whatwg.org/">CSS Figures</a> has evolved into a different approach.
 	</dl>
 
 	Issue: Add final content from previous level