[DOCS][Release notes][9.3.x] Upgrading to 9.3.x fails when a rule action contains oversized content (elastic#269818)

## Summary

Contributes to
elastic/docs-content-internal#1196 by adding
known issue about upgrade failures when users upgrade to 9.3.x with a
rule action containing values exceeding 32766 bytes. This PR updates
Kibana release notes. The Observability and Security release notes are
being updated via elastic/docs-content#6554.

Author: nastasha-solomon

docs/release-notes/known-issues.md

@@ -24,6 +24,27 @@ When performing bulk agent actions across all pages, select agents individually
 
 This issue is resolved in {{stack}} 9.2.8, 9.3.3, 9.4.0. Version 9.1 is end of life — upgrade to {{stack}} 9.2.8+ or 9.3.3+.
 
+::::
+
+::::{dropdown} Upgrading to 9.3.x fails when a rule action contains oversized content
+
+Applies to: {{stack}} 9.3.0, 9.3.1, 9.3.2, 9.3.3, 9.3.4
+
+**Details**
+
+Upgrading from 9.2.x to 9.3.x can fail if any rule (Stack, Observability, or Security) has a connector action whose parameter values are larger than 32,766 bytes. Common examples include email message bodies or HTML templates, large webhook payloads, or Slack messages built from verbose templates.
+
+During the upgrade, {{kib}} migrates rule saved objects to a new internal mapping. Any oversized action parameter value causes the migration to abort with an error similar to:
+
+````
+Flattened field [alert.actions.params] contains one immense field whose keyed encoding is longer than the allowed max length of 32766 bytes
+````
+
+**Workaround**
+
+If the upgrade has failed with this error, identify rules that use connectors with large content (particularly email, webhook, and Slack connectors) and shorten the action parameter values, such as message bodies or HTML templates. Then retry the upgrade.
+
+
 ::::
 
 ::::{dropdown} Stack alerts remain active instead of transitioning to recovered