[8.3] Document the behavior of IM rules and multi-value indicator documents (backport #4326)

[mergify]

This is an automatic backport of pull request #4326 done by Mergify.
Cherry-pick of ed4d816 has failed:

On branch mergify/bp/8.3/pr-4326
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	both modified:   docs/detections/rules-ui-create.asciidoc

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

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[github-actions]

Documentation previews:

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[nastasha-solomon]

@elasticmachine run elasticsearch-ci/docs