docs: document why callbackDispatcher daemon threads are safe for offset bookkeeping

Author: yeikel

src/main/java/io/confluent/connect/elasticsearch/ElasticsearchClient.java

@@ -145,6 +145,8 @@ public ElasticsearchClient(
 
     AtomicInteger dispatcherCounter = new AtomicInteger(0);
     String dispatcherPrefix = connectorName + "-" + taskId + "-es-bulk-callback-";
+    // Daemon: BulkIngester.close() blocks until all afterBulk callbacks complete, so the
+    // JVM never exits with offset bookkeeping mid-flight on these threads.
     this.callbackDispatcher = Executors.newCachedThreadPool(r -> {
       Thread t = new Thread(r, dispatcherPrefix + dispatcherCounter.getAndIncrement());
       t.setDaemon(true);