Port #4418 to serverless

[dedemorton]

Description

Adds documentation about "no data" setting for custom threshold rule.

Documentation sets edited in this PR

Check all that apply.

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

Related issue

Closes #4429.

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• 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) (changes already applied to 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>

[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
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.

[elasticdocs]

🚀 Built elastic-dot-co-docs-preview-docs successfully!

• https://elastic-dot-co-docs-production-c16nzt51f-elastic-dev.vercel.app

• Built with commit 111c243

Issues? Visit #next-docs in Slack

[elasticdocs]

🚀 Built elastic-dot-co-docs-preview-docs successfully!

• https://elastic-dot-co-docs-production-dkw6pchem-elastic-dev.vercel.app

• Built with commit 111c243

Issues? Visit #next-docs in Slack

[alaudazzi]

LGTM