[docs][elasticsearch] - add a warning regarding multiple clusters

[VihasMakwana]

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

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/auditbeat/elasticsearch-output.md
• docs/reference/filebeat/elasticsearch-output.md
• docs/reference/heartbeat/elasticsearch-output.md
• docs/reference/metricbeat/elasticsearch-output.md
• docs/reference/packetbeat/elasticsearch-output.md
• docs/reference/winlogbeat/elasticsearch-output.md