MINOR: Add comment to onPartitionsLost override (#14121)

This adds comments to the ConsumerRebalanceListener overrides, in order to briefly explain why we are overriding these methods, when they are called, and what you can or can't do. Especially onPartitionsLost can create some confusion given the default implementation.

Reviewers: Luke Chen <showuon@gmail.com>, David Jacot <djacot@confluent.io>

examples/src/main/java/kafka/examples/Consumer.java

@@ -151,15 +151,20 @@ public class Consumer extends Thread implements ConsumerRebalanceListener {
     @Override
     public void onPartitionsRevoked(Collection<TopicPartition> partitions) {
         Utils.printOut("Revoked partitions: %s", partitions);
+        // this can be used to commit pending offsets when using manual commit and EOS is disabled
     }
 
     @Override
     public void onPartitionsAssigned(Collection<TopicPartition> partitions) {
         Utils.printOut("Assigned partitions: %s", partitions);
+        // this can be used to read the offsets from an external store or some other initialization
     }
 
     @Override
     public void onPartitionsLost(Collection<TopicPartition> partitions) {
         Utils.printOut("Lost partitions: %s", partitions);
+        // this is called when partitions are reassigned before we had a chance to revoke them gracefully
+        // we can't commit pending offsets because these partitions are probably owned by other consumers already
+        // nevertheless, we may need to do some other cleanup
     }
 }