[8.x](backport #4501) Add reference to prefer-ilm setting for apm lifeycle known bug

[mergify]

Description

Clarify that one of the suggested fix for the unmanaged indices bug introduced in 8.15.0 and fixed in 8.15.1 should only be applied for datatreams with default policies (as they are the only ones affected). If the suggested fix was applied to a datastream using custom ILM policy then the prefer_ilm setting needs to be added to the custom ILM policy.

Documentation sets edited in this PR

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• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

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Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

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• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4501 done by [Mergify](https://mergify.com).

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