Update env variables for ES authentication (#1356)

This is to bring the docs in line with the change in
elastic/elastic-agent#5536.

Author: swiatekm

docs/en/ingest-management/elastic-agent/configuration/env/container-envs.asciidoc

@@ -38,6 +38,8 @@ include::shared-env.asciidoc[tag=elasticsearch-username]
 
 include::shared-env.asciidoc[tag=elasticsearch-password]
 
+include::shared-env.asciidoc[tag=elasticsearch-api-key]
+
 include::shared-env.asciidoc[tag=elasticsearch-ca]
 
 include::shared-env.asciidoc[tag=kibana-host]

docs/en/ingest-management/elastic-agent/configuration/env/shared-env.asciidoc

@@ -421,7 +421,7 @@ Setting this to `true` is not recommended.
 
 // To do: link to required privileges
 
-*Default:* `elastic`
+*Default:* none
 
 // end::elasticsearch-username[]
 
@@ -450,12 +450,25 @@ This user needs the privileges required to publish events to {es}.
 
 | (string) The basic authentication password used to connect to {kib} and retrieve a `service_token` for {fleet}.
 
-*Default:* `changeme`
+*Default:* none
 
 // end::elasticsearch-password[]
 
 // =============================================================================
 
+// tag::elasticsearch-api-key[]
+|
+[id="env-{type}-elasticsearch-api-key"]
+`ELASTICSEARCH_API_KEY`
+
+| (string) API key used for authenticating to Elasticsearch.
+
+*Default:* none
+
+// end::elasticsearch-api-key[]
+
+// =============================================================================
+
 // tag::es-password[]
 |
 [id="env-{type}-es-password"]