KAFKA-18674: Document the incompatible changes in parsing --bootstrap-server

docs/upgrade.html

@@ -85,6 +85,8 @@ <h5><a id="upgrade_400_notable" href="#upgrade_400_notable">Notable changes in 4
                         <li>The default properties files for KRaft mode are no longer stored in the separate <code>config/kraft</code> directory since Zookeeper has been removed. These files have been consolidated with other configuration files.
                             Now all configuration files are in <code>config</code> directory.
                         </li>
+                        <li>The validation of <code>--bootstrap-server</code> has become stricter. Providing space-separated bootstrap servers (e.g., "broker1:9092 broker2:9092 broker3:9092") instead of comma-separated (e.g., "broker1:9092,broker2:9092,broker3:9092") will result in an exception.
+                        </li>
                     </ul>
                 </li>
                 <li><b>Broker</b>