Add note on APM Server Personal Data

[ericywl]

Description

Add other backports for #4825.

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

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>

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It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.