[DOCS][ResponseOps][Reporting][9.1 & Serv] Add documentation about required privileges with API key authentication (elastic#2885)

Contributes to elastic#2679 by
adding a note about the new API authentication key method that was
implemented for report generation in 8.19. The note also explains the
specific cluster and index privileges that users with cross-cluster
search environments must have.

Previews:
- [Automatically generate
reports](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/2885/explore-analyze/report-and-share/automating-report-generation)
- [Configure reporting in Kibana | Configuration
overview](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/2885/deploy-manage/kibana-reporting-configuration#configuration-overview)

**Corresponding 8.19 doc updates:**
elastic/kibana#234255

Author: nastasha-solomon

deploy-manage/kibana-reporting-configuration.md

@@ -36,6 +36,11 @@ Before upgrading {{kib}} in a production environment, we encourage you to test y
 
 To secure {{report-features}}, you must grant users access to reporting functionality and protect the reporting endpoints with TLS/SSL encryption. Additionally, you can install graphical packages on the operating system to enable screenshot capabilities in the {{kib}} server.
 
+:::{note}
+:applies_to: {stack: ga, serverless: unavailable}
+API keys are used to authenticate requests to generate reports. If you have a cross-cluster search environment and want to generate reports from remote clusters, you must have the appropriate cluster and index privileges on the remote cluster and local cluster. For example, if requests are authenticated with an API key, the API key requires certain privileges on the local cluster that contains the leader index, instead of the remote. For more information and examples, refer to [Configure roles and users for remote clusters](../deploy-manage/remote-clusters/remote-clusters-cert.md#remote-clusters-privileges-cert).
+:::
+
 Configuring reporting in your environment involves two main areas:
 
 ### Granting users access to {{report-features}}

explore-analyze/report-and-share/automating-report-generation.md

@@ -12,6 +12,11 @@ products:
 
 To automatically generate PDF and CSV reports, generate a POST URL, then submit an HTTP `POST` request using {{watcher}} or a script. In {{stack}} 9.1 and Serverless, you can use {{kib}} to generate reports on a recurring schedule and share them with a list of emails that you specify.
 
+:::{note}
+:applies_to: {stack: ga, serverless: unavailable}
+API keys are used to authenticate requests to generate reports. If you have a cross-cluster search environment and want to generate reports from remote clusters, you must have the appropriate cluster and index privileges on the remote cluster and local cluster. For example, if requests are authenticated with an API key, the API key requires certain privileges on the local cluster that contains the leader index, instead of the remote. For more information and examples, refer to [Configure roles and users for remote clusters](../../deploy-manage/remote-clusters/remote-clusters-cert.md#remote-clusters-privileges-cert).
+:::
+
 ## Create a POST URL [create-a-post-url]
 
 Create the POST URL that triggers a report to generate PDF and CSV reports.