[8.x] Update rules-ui-create.asciidoc - fallback behavior in timestamp overrides (backport #6425)

[mergify]

Small PR to explicitly state the fallback behavior on timestamp overrides.
It is currently implied with the mention of the Do not use @timestamp as a fallback timestamp field option

---

This is an automatic backport of pull request #6425 done by Mergify.

[mergify]

Cherry-pick of e9f0d81 has failed:

On branch mergify/bp/8.x/pr-6425
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit e9f0d81a.
  (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/detections/rules-ui-create.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/rules/rules-ui-create.asciidoc

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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