Mention logs in Elasticsearch integration troubleshooting

This wasn't really called out sufficiently on this page and recently
we've learnt these logs are pretty much the quickest way to spot
problems so we should be highlighting this at the top of the
Troubleshooting section.

doc/integration/elasticsearch.md

@@ -761,6 +761,17 @@ However, some larger installations may wish to tune the merge policy settings:
 
 ## Troubleshooting
 
+One of the most valuable tools for identifying issues with the Elasticsearch
+integration will be logs. The most relevant logs for this integration are:
+
+1. [`sidekiq.log`](../administration/logs.md#sidekiqlog) - All of the
+   indexing happens in Sidekiq, so much of the relevant logs for the
+   Elasticsearch integration can be found in this file.
+1. [`elasticsearch.log`](../administration/logs.md#elasticsearchlog) - There
+   are additional logs specific to Elasticsearch that are sent to this file
+   that may contain useful diagnostic information about searching,
+   indexing or migrations.
+
 Here are some common pitfalls and how to overcome them.
 
 ### How can I verify that my GitLab instance is using Elasticsearch?