[8.19](backport #47791) [docs][elasticsearch] - add a warning regarding multiple clusters

[mergify]

Proposed commit message

Adds a warning to the Elasticsearch output docs stating that all hosts must be part of the same cluster.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Screenshot

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This is an automatic backport of pull request #47791 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 56a35a9 has failed:

On branch mergify/bp/8.19/pr-47791
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Unmerged paths:
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	deleted by us:   docs/reference/auditbeat/elasticsearch-output.md
	deleted by us:   docs/reference/filebeat/elasticsearch-output.md
	deleted by us:   docs/reference/heartbeat/elasticsearch-output.md
	deleted by us:   docs/reference/metricbeat/elasticsearch-output.md
	deleted by us:   docs/reference/packetbeat/elasticsearch-output.md
	deleted by us:   docs/reference/winlogbeat/elasticsearch-output.md

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[elasticmachine]

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