[Known Issue] Add docs to describe a known issue/limitation of EQL rule cross-cluster search (backport #4813)

[mergify]

This is an automatic backport of pull request #4813 done by Mergify.
Cherry-pick of 01ec37b has failed:

On branch mergify/bp/8.4/pr-4813
Your branch is up to date with 'origin/8.4'.

You are currently cherry-picking commit 01ec37b9.
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Unmerged paths:
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	both modified:   docs/detections/api/rules/rules-api-create.asciidoc
	both modified:   docs/detections/rules-ui-create.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

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