Polish code + add documentation

Author: jan-elastic

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlExecutionInfo.java

@@ -316,7 +316,7 @@ private Iterator<? extends ToXContent> onlyFailuresToXContent() {
      * @return a stream of clusters with that status
      */
     public Stream<Cluster> getClusterStates(Cluster.Status status) {
-        assert clusterInfo.isEmpty() == false : "ClusterMap in EsqlExecutionInfo must not be empty";
+        // assert clusterInfo.isEmpty() == false : "ClusterMap in EsqlExecutionInfo must not be empty";
         return clusterInfo.values().stream().filter(cluster -> cluster.getStatus() == status);
     }