Obs AI Assistant supports other models than ELSER when field type semantic_text is used

[mdbirnstiehl]

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>

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[mdbirnstiehl]

@sorenlouv could you take a look here and see if I'm missing anything. Thanks!

[sorenlouv]

I've added some notes on how to use semantic text fields in search connectors. I'd expect the search connector docs to also cover how to add semantic text fields.

[colleenmcginnis]

A few minor suggestions below!