Update warning re ATOMIC_REBUILD on Elasticsearch >= 5.4

It appears that the bug may have reappeared in 5.5 final. Fixes #3985

docs/topics/search/backends.rst

@@ -45,7 +45,7 @@ If you have disabled auto update, you must run the :ref:`update_index` command o
 ==================
 
 .. warning::
-    This option is not compatible with Elasticsearch 5.4.x due to `a bug in the handling of aliases <https://github.com/elastic/elasticsearch/issues/24644>`_ in this release. Please use a 5.3.x or 5.5.x release instead.
+    This option may not work on Elasticsearch version 5.4 and above, due to `a bug in the handling of aliases <https://github.com/elastic/elasticsearch/issues/24644>`_ affecting these releases.
 
 By default (when using the Elasticsearch backend), when the ``update_index`` command is run, Wagtail deletes the index and rebuilds it from scratch. This causes the search engine to not return results until the rebuild is complete and is also risky as you can't rollback if an error occurs.