[AI Assistant] Add Non-string fields anonymization limitation (#4960)

This PR closes [Issue 3381](elastic/docs-content#3381)

Author: mdbirnstiehl

docs/en/observability/observability-ai-assistant.asciidoc

@@ -517,11 +517,11 @@ Anonymization requires the following:
 
 Anonymization has the following limitations:
 
+* *Non-string fields*: Anonymization only applies to string values. Booleans, numbers, image types, and other non-string values are ignored.
 * *Performance (NER)*: Running an NER model can add latency depending on the request. To improve performance of the model, consider scaling up your ML nodes by adjusting deployment parameters: increase `number_of_allocations` for better throughput and `threads_per_allocation` for faster individual requests. For details, refer to https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-ml-start-trained-model-deployment[start trained model deployment API].
 * *Structured JSON*: The NER model we validated (`elastic/distilbert-base-uncased-finetuned-conll03-english`) is trained on natural English text and often misses entities inside JSON or other structured data. If thorough masking is required, prefer regex rules and craft them to account for JSON syntax.
 * *False negatives / positives*: No model or pattern is perfect. Model accuracy may vary depending on model and input.
 
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 [discrete]
 [[obs-ai-known-issues]]
 == Known issues