Clarifications added to remote clusters, self-managed, api key based auth

[eedugon]

This is a preliminary / quick solution to address the issues and noise caused by the lack of details of the doc to configure remote clusters from a self-managed cluster towards ECK.

This PR improves:

• The note that was clearly vague, telling users that enabling the server interface and adding xpack.security.remote_cluster_client.ssl.certificate_authorities is NOT needed when remote is ECH.
• Highlights at the beginning that the remote cluster server interface port is 9443.
• Tells the user to NOT try sniff mode if the remote is on ECH, ECE or ECK, as that will never be an option.

In a next iteration we will refine the content to cover other remote possibilities, such as ECE or ECK, completely, as we have in other docs.

For example for self-manged --> ECE we will require the user to add the ECE proxy CA into the self-managed list of trusted certs, but that's not covered yet.

Related issues and SDHs:
https://github.com/elastic/docs-content-internal/issues/241
https://github.com/elastic/sdh-elasticsearch/issues/9166
https://github.com/elastic/sdh-elasticsearch/issues/8951

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/remote-clusters/remote-clusters-api-key.md

[yetanothertw]

Approving to unblock you, @eedugon!

I've left a few minor suggestions, but am afraid don't know enough about this subject to contribute more meaningfully. Many thanks!

[shainaraskas]

great quick wins. nice