[Docs] Add known issue about Obs AI Assistant KB to 9.0 docs (#220401)

^^ 

Counterpart of https://github.com/elastic/kibana/pull/220397 for 9.0
docs

docs/release-notes/known-issues.md

@@ -6,6 +6,61 @@ navigation_title: "Known issues"
 
 ## 9.0.0
 
+::::{dropdown} Observability AI assistant Knowledge Base entries with empty text can lead to Kibana OOM or restarts
+:name:known-issue-220339
+
+**Details** 
+
+The semantic text migration can cause excessive traffic to a cluster and might eventually cause the Kibana instance to crash due to OOM, together with increase of requests to Elasticsearch & ML nodes.
+
+The problem can occur when there is one or more empty text Knowledge Base documents.
+
+The migration script does not handle this scenario and will indefinitely update the same document.
+
+Because the document update involves semantic_text an ML node is kept warm further increasing the costs.
+
+The issue involves semantic_text field type (and thus the semantic_text migration which is causing this issue), introduced in the knowledge base feature in 8.17.
+
+**Workaround** 
+
+1. Pause the Kibana instance if possible. If not possible, skip this step.
+2. Run a dry run query to identify if you have empty Knowledge Base documents. If you have at least 1 hit, you can be affected by the problem.
+
+    ```sh
+    GET .kibana-observability-ai-assistant-kb/_search
+    {
+      "query": {
+        "bool": {
+          "must": [{ "exists": { "field": "text" }}],
+          "must_not": [ { "wildcard": { "text": "*" } }
+          ]
+        }
+      }
+    }
+    ```
+
+3. Execute the deletion. For extra safety, you might want to trigger a snapshot before executing it.
+
+    ```sh
+    POST .kibana-observability-ai-assistant-kb/_delete_by_query
+    {
+      "query": {
+        "bool": {
+          "must": [{ "exists": { "field": "text" }}],
+          "must_not": [ { "wildcard": { "text": "*" } }
+          ]
+        }
+      }
+    }
+    ```
+
+For more information, check:
+
+- [#220339](https://github.com/elastic/kibana/issues/220339)
+- [#220342](https://github.com/elastic/kibana/issues/220342)
+
+::::
+
 ::::{dropdown} Issue with follower indices during upgrade
 :name: ua-follower-indices