Mention plugins in troubleshooting Elasticsearch integration

This comes after a recent issue was diagnosed for one of our customers
where the plugin was causing the integration not to work. Hopefully
calling out plugins in our docs may save some time as customers may be
able to spot early on that the plugin is the thing causing issues before
asking us to diagnose the problem. Since there are many plugins that are
3rd party to Elasticsearch and they can significantly change the
behaviour of Elasticsearch it's impossible for us to test against all
plugins and be sure what does and does not work.

doc/integration/elasticsearch.md

@@ -918,6 +918,13 @@ Gitlab::Elastic::Indexer::Error: time="2020-01-23T09:13:00Z" level=fatal msg="he
 You probably have not used either `http://` or `https://` as part of your value in the **"URL"** field of the Elasticsearch Integration Menu. Please make sure you are using either `http://` or `https://` in this field as the [Elasticsearch client for Go](https://github.com/olivere/elastic) that we are using [needs the prefix for the URL to be accepted as valid](https://github.com/olivere/elastic/commit/a80af35aa41856dc2c986204e2b64eab81ccac3a).
 Once you have corrected the formatting of the URL, delete the index (via the [dedicated Rake task](#gitlab-advanced-search-rake-tasks)) and [reindex the content of your instance](#enabling-advanced-search).
 
+### My Elasticsearch cluster has a plugin and the integration is not working
+
+Certain 3rd party plugins may introduce bugs in your cluster or for whatever
+reason may be incompatible with our integration. You should try disabling
+plugins so you can rule out the possibility that the plugin is causing the
+problem.
+
 ### Low-level troubleshooting
 
 There is a [more structured, lower-level troubleshooting document](../administration/troubleshooting/elasticsearch.md) for when you experience other issues, including poor performance.