[8.13](backport #3953) Clarify descriptions of APM indicators for SLOs

[mergify[bot]]

Closes #3913

@graphaelli @grabowskit WDYT? Is this what you had in mind?

If you want me to add a note about the impact of unknown outcomes, I can do that, but I need to know what the impact is. :-)

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This is an automatic backport of pull request #3953 done by Mergify.

[mergify[bot]]

Cherry-pick of 4770b0c9c69e294fd599a9bd5f3c21a5df0c9f5e has failed:

On branch mergify/bp/8.13/pr-3953
Your branch is up to date with 'origin/8.13'.

You are currently cherry-picking commit 4770b0c9.
  (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/en/observability/slo-create.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
    deleted by us:   docs/en/serverless/slos/create-an-slo.mdx

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

[github-actions[bot]]

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[mergify[bot]]

This pull request has not been merged yet. Could you please review and merge it @dedemorton? 🙏

[mergify[bot]]

This pull request has not been merged yet. Could you please review and merge it @dedemorton? 🙏