Add known issue for orphaned status blocking upgrades

[michel-laterman]

As reported by elastic/elastic-agent#6213, if the audit/unenroll endpoint is used, agent status becomes inconsistent and certain actions, such as UPGRADE can no longer be issued to them through the fleet-ui.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

Thanks a lot for adding this @michel-laterman! Looks great. I took the liberty of committing my one small suggestion (x4).

LGTM! 🚀