Harmonise with other gitlab_rails['env'] settings

See https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/c1daa21ae753588cce46077dfa668ae195eb1db6/files/gitlab-config-template/gitlab.rb.template#L480-484 and https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/c1daa21ae753588cce46077dfa668ae195eb1db6/files/gitlab-config-template/gitlab.rb.template#L527-531

doc/administration/troubleshooting/elasticsearch.md

@@ -261,6 +261,9 @@ Beyond that, you will want to review the error. If it is:
 - Specifically from the indexer, this could be a bug/issue and should be escalated to
   GitLab support.
 - An OS issue, you will want to reach out to your systems administrator.
+- A `Faraday::TimeoutError (execution expired)` error **and** you're using a proxy,
+  [set a custom  `gitlab_rails['env']` environment variable, called `no_proxy`](https://docs.gitlab.com/omnibus/settings/environment-variables.html)
+  with the IP address of your Elasticsearch host.
 
 ### Troubleshooting performance