Add DistributedPubSub message guarantee in doc (Fixes #21441) (#21713)

akka-docs/rst/java/distributed-pub-sub.rst

@@ -179,6 +179,15 @@ and then it takes a while for it to be populated.
 
    akka.extensions = ["akka.cluster.pubsub.DistributedPubSub"]
 
+Delivery Guarantee
+------------------
+
+As in :ref:`message-general-rules` of Akka, message delivery guarantee in distributed pub sub modes is **at-most-once delivery**.
+In other words, messages can be lost over the wire.
+
+If you are looking for at-least-once delivery guarantee, we recommend `Kafka Akka Streams integration <https://github.com/akka/reactive-kafka>`_.
+
+
 Dependencies
 ------------