Add note about reindex API to the reindex helper

Author: honzakral

elasticsearch/helpers/__init__.py

@@ -327,6 +327,12 @@ def reindex(client, source_index, target_index, query=None, target_client=None,
  to another, potentially (if `target_client` is specified) on a different cluster.
  If you don't specify the query you will reindex all the documents.
 
+ Since ``2.3`` a :meth:`~elasticsearch.Elasticsearch.reindex` api is
+ available as part of elasticsearch itself. It is recommended to use the api
+ instead of this helper wherever possible. The helper is here mostly for
+ backwards compatibility and for situations where more flexibility is
+ needed.
+
  .. note::
 
  This helper doesn't transfer mappings, just the data.