Merge branch 'add-docs-to-rake-cluster-reindexing' into 'master'

Add docs for rake gitlab:elastic:reindex_cluster

See merge request gitlab-org/gitlab!35723

doc/integration/elasticsearch.md

@@ -433,6 +433,7 @@ The following are some available Rake tasks:
 | [`sudo gitlab-rake gitlab:elastic:recreate_index[<TARGET_NAME>]`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/tasks/gitlab/elastic.rake)     | Wrapper task for `gitlab:elastic:delete_index[<TARGET_NAME>]` and `gitlab:elastic:create_empty_index[<TARGET_NAME>]`.                                                                       |
 | [`sudo gitlab-rake gitlab:elastic:index_snippets`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/tasks/gitlab/elastic.rake)                   | Performs an Elasticsearch import that indexes the snippets data.                                                                                                                          |
 | [`sudo gitlab-rake gitlab:elastic:projects_not_indexed`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/tasks/gitlab/elastic.rake)             | Displays which projects are not indexed.                                                                                                                                                  |
+| [`sudo gitlab-rake gitlab:elastic:reindex_cluster`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/tasks/gitlab/elastic.rake)             | Schedules a zero-downtime cluster reindexing task. This feature should be used with an index that was created after GitLab 13.0. |
 
 NOTE: **Note:**
 The `TARGET_NAME` parameter is optional and will use the default index/alias name from the current `RAILS_ENV` if not set.