Update javadoc

Author: jasontedor

server/src/main/java/org/elasticsearch/index/shard/GlobalCheckpointListeners.java

@@ -83,7 +83,8 @@ public interface GlobalCheckpointListener {
      * Add a global checkpoint listener. If the global checkpoint is above the current global checkpoint known to the listener then the
      * listener will be asynchronously notified on the executor used to construct this collection of global checkpoint listeners. If the
      * shard is closed then the listener will be asynchronously notified on the executor used to construct this collection of global
-     * checkpoint listeners.
+     * checkpoint listeners. The listener will only be notified of at most one event, either the global checkpoint is updated or the shard
+     * is closed. A listener must re-register after one of these events to receive subsequent events.
      *
      * @param currentGlobalCheckpoint the current global checkpoint known to the listener
      * @param listener                the listener