[8.9](backport #3687) [DOCS] Clarify AI Assistant will spawn ML nodes

[mergify]

Clarify AI Assistant will spawn ML nodes

To the doc writers:

• please change wording
• backport to all versions back to AI Assistant pages got created
• add an additional paragraph if the use of AI Assistant / Knowledge Base implies ML Nodes will no more be scaled down back to 0

  ---

  This is an automatic backport of pull request [DOCS] Clarify AI Assistant will spawn ML nodes #3687 done by Mergify.

[mergify]

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

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