[8.16](backport #4349) Update threshold-alert.asciidoc

[mergify]

Hey, team, I'm sending this pull request because a customer asked about the bars from the preview chart. It was only showing 3 bars instead of 4, as the customer was expecting to see. However, this is hardcoded and will only show the top 3.

Description

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 #

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)
  • 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: [Request]: Investigate and port #4349 to serverless if needed #4417 (we need to validate that this needs to be backported)

---

This is an automatic backport of pull request #4349 done by [Mergify](https://mergify.com).

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