[8.6] Document a known issue about detection rules being duplicated upon update (backport #4022)

[mergify]

This is an automatic backport of pull request #4022 done by Mergify.
Cherry-pick of 254cc02 has failed:

On branch mergify/bp/8.6/pr-4022
Your branch is up to date with 'origin/8.6'.

You are currently cherry-picking commit 254cc025.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/release-notes/8.6.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/release-notes/8.7.asciidoc
	deleted by us:   docs/release-notes/8.8.asciidoc
	deleted by us:   docs/release-notes/8.9.asciidoc

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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