Update changelog

Author: albertzaharovits

docs/changelog/129464.yaml

@@ -5,7 +5,7 @@ type: deprecation
 issues: []
 deprecation:
   title: Deprecate `indices.merge.scheduler.use_thread_pool` setting
-  area: Engine
-  details: Please describe the details of this change for the release notes. You can
-    use asciidoc.
-  impact: Please describe the impact of this change to users
+  area: Ingest
+  details: This deprecates the `indices.merge.scheduler.use_thread_pool` node setting that was introduced in #120869.
+    This setting should not normally be used, unless instructed so by engineering to get around temporary issues with the new threadpool-based merge scheduler.
+  impact: There should be no impact to users since the setting was not released before its deprecation here (and is not documented).