Add default Elasticsearch credentials to docs (#72617)

docs/developer/advanced/running-elasticsearch.asciidoc

@@ -13,6 +13,10 @@ This will run a snapshot of {es} that is usually built nightly. Read more about
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 yarn es snapshot
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+By default, two users are added to Elasticsearch:
+
+  - A superuser with username: `elastic` and password: `changeme`, which can be used to log into Kibana with.
+  - A user with username: `kibana_system` and password `changeme`. This account is used by the Kibana server to authenticate itself to Elasticsearch, and to perform certain actions on behalf of the end user. These credentials should be specified in your kibana.yml as described in <<using-kibana-with-security>>
 
 See all available options, like how to specify a specific license, with the `--help` flag.
 
@@ -115,4 +119,4 @@ PUT _cluster/settings
 }
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-Follow the cross-cluster search instructions for setting up index patterns to search across clusters (<<management-cross-cluster-search>>).
+Follow the cross-cluster search instructions for setting up index patterns to search across clusters (<<management-cross-cluster-search>>).