Update ES indexer docs for Omnibus users

doc/integration/elasticsearch.md

@@ -42,7 +42,11 @@ use the packages that are available for your OS.
 In order to improve elasticsearch indexing performance, GitLab has made available a [new indexer written in Go](https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer).
 This will replace the included Ruby indexer in the future but should be considered beta software for now, so there may be some bugs.
 
-If you would like to use it, please follow the instructions below.
+The Elasticsearch Go indexer is included in Omnibus for GitLab 11.8 and newer.
+
+To use the new Elasticsearch indexer included in Omnibus, check the box "Use the new repository indexer (beta)"  when [enabling the Elasticsearch integration](#enabling-elasticsearch).
+
+If you would like to use the Elasticsearch Go indexer with a source installation or an older version of GitLab, please follow the instructions below.
 
 ### Installation