[7.17] [DOCS] Update Fleet Server guide - Fleet Server keeps using the ES host provided at install/enroll time. (backport #1185)

[mergify]

Feedback from real users.

Due to the limitation elastic/elastic-agent#2784 (https://support.elastic.co/knowledge/f69b6e46), Fleet Server still keeps using the Elasticsearch host provided at enroll time instead of using the one in the policy.

Elastic agent & Fleet are working on overcoming the limitation via elastic/fleet-server#3464 but we should add a big warning message about this.

This message should be backported to all versions, at least until we'll introduce the enhancements to take into account the ES hosts coming from the policy.

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This is an automatic backport of pull request #1185 done by Mergify.

[mergify]

Cherry-pick of 7257560 has failed:

On branch mergify/bp/7.17/pr-1185
Your branch is up to date with 'origin/7.17'.

You are currently cherry-picking commit 7257560b.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	deleted by us:   docs/en/ingest-management/fleet/add-fleet-server-on-prem.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[kilfoyle]

Closing unmerged. Looks like this file didn't exist in 7.17.