Document breaking change in ClusterHealthResponse in 2.2

docs/reference/migration/migrate_2_2.asciidoc

@@ -43,6 +43,21 @@ changed to now route standard output to the journal and standard error
 to inherit this setting (these are the defaults for systemd). These
 settings can be modified by editing the `elasticsearch.service` file.
 
+[float]
+=== Java Client
+
+Previously it was possible to iterate over `ClusterHealthResponse` to get information about `ClusterIndexHealth`.
+While this is still possible, it requires now iterating over the values returned from `getIndices()`:
+
+[source,java]
+---------------
+ClusterHealthResponse clusterHealthResponse = client.admin().cluster().prepareHealth().get();
+for (Map.Entry<String, ClusterIndexHealth> index : clusterHealthResponse.getIndices().entrySet()) {
+    String indexName = index.getKey();
+    ClusterIndexHealth health = index.getValue();
+}
+---------------
+
 [float]
 === Cloud AWS Plugin