[8.17](backport #4754) Obs AI Assistant supports other models than ELSER when field type semantic_text is used

[mergify]

Description

The Obs AI Assistant supports other models than ELSER when field type semantic_text is used and the docs need to be updated to show this.

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 #4608

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 #4754 done by [Mergify](https://mergify.com).

[mergify]

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Unmerged paths:
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	both modified:   docs/en/observability/observability-ai-assistant.asciidoc

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