[8.19] [Docs] Add known issue about AI Obs Assistant KB documents (#220397) (#220402)

# Backport

This will backport the following commits from `8.18` to `8.19`:
- [[Docs] Add known issue about AI Obs Assistant KB documents
(#220397)](#220397)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT
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Add known issue about AI Obs Assistant KB documents (#220397)\n\n^^
\n\nLet me know if we should add more links to more context, etc. \nI
added the entry to every minor release notes section of 8.18 and
8.17\nminor will create a different PR to the new docs system to
document this\nfor 9.0.\n\n<img width=\"869\"
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Add known issue about AI Obs Assistant KB
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Add known issue about AI Obs Assistant KB documents (#220397)\n\n^^
\n\nLet me know if we should add more links to more context, etc. \nI
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8.17\nminor will create a different PR to the new docs system to
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Add known issue about AI Obs Assistant KB documents (#220397)\n\n^^
\n\nLet me know if we should add more links to more context, etc. \nI
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BACKPORT-->

Author: florent-leborgne

docs/CHANGELOG.asciidoc

@@ -100,6 +100,12 @@ include::upgrade-notes.asciidoc[]
 
 The 8.18.1 release includes the following enhancements and fixes.
 
+[float]
+[[known-issues-8.18.1]]
+=== Known issues
+
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[enhancement-v8.18.1]]
 === Enhancements
@@ -140,6 +146,62 @@ Review the following information about the {kib} 8.18.0 release.
 [[known-issues-8.18.0]]
 === Known issues
 
+// tag::known-issue-220339[]
+.Observability AI assistant Knowledge Base entries with empty text can lead to Kibana OOM or restarts.
+[%collapsible]
+====
+*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:
+
+* https://github.com/elastic/kibana/issues/220339[#220339]
+* https://github.com/elastic/kibana/issues/220342[#220342]
+
+====
+// end::known-issue-220339[]
+
 // tag::known-issue-211850[]
 .Upgrade Assistant - Rollup jobs need to be stopped before rollup indices are reindexed.
 [%collapsible]
@@ -409,6 +471,12 @@ Machine Learning::
 
 The 8.17.6 release includes the following enhancements and fixes.
 
+[float]
+[[known-issues-8.17.6]]
+=== Known issues
+
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[enhancement-v8.17.6]]
 === Enhancements
@@ -449,6 +517,12 @@ Management::
 
 The 8.17.5 release includes the following enhancements and fixes.
 
+[float]
+[[known-issues-8.17.5]]
+=== Known issues
+
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[enhancement-v8.17.5]]
 === Enhancements
@@ -494,6 +568,12 @@ Management::
 
 The 8.17.4 release includes the following fixes.
 
+[float]
+[[known-issues-8.17.4]]
+=== Known issues
+
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[fixes-v8.17.4]]
 === Fixes
@@ -513,6 +593,12 @@ Management::
 == {kib} 8.17.3
 The 8.17.3 release includes the following bug fixes.
 
+[float]
+[[known-issues-8.17.3]]
+=== Known issues
+
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[fixes-v8.17.3]]
 === Bug fixes
@@ -534,6 +620,12 @@ Sharing::
 == {kib} 8.17.2
 The 8.17.2 release includes the following bug fixes.
 
+[float]
+[[known-issues-8.17.2]]
+=== Known issues
+
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[fixes-v8.17.2]]
 === Bug fixes
@@ -554,6 +646,8 @@ The 8.17.1 release includes the following known issues, enhancements, and bug fi
 
 include::CHANGELOG.asciidoc[tag=known-issue-206400]
 
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 [float]
 [[enhancement-v8.17.1]]
 === Enhancements
@@ -589,6 +683,8 @@ The 8.17.0 release includes the following known issues.
 
 include::CHANGELOG.asciidoc[tag=known-issue-187254]
 
+include::CHANGELOG.asciidoc[tag=known-issue-220339]
+
 // tag::known-issue-206400[]
 .Upgrade Assistant displays configured source mode in mappings as critical deprecation issue.
 [%collapsible]