WAK: add note in javadoc about missing feature.

core/src/main/java/org/bitcoinj/kits/WalletAppKit.java

@@ -131,7 +131,8 @@ public class WalletAppKit extends AbstractIdleService {
 
     /**
      * If you want to learn about the sync process, you can provide a listener here. For instance, a
-     * {@link DownloadListener} is a good choice.
+     * {@link DownloadListener} is a good choice. This has no effect unless setBlockingStartup(false) has been called
+     * too, due to some missing implementation code.
      */
     public WalletAppKit setDownloadListener(PeerEventListener listener) {
         this.downloadListener = listener;