add details to changelog

Author: valeriy42

docs/changelog/121667.yaml

@@ -6,6 +6,7 @@ issues: []
 deprecation:
   title: Add deprecation warning for flush API
   area: Machine Learning
-  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
+  details: The anomaly detection job flush API is deprecated since it is only required for the post data API,
+    which was deprecated since 7.11.0.
+  impact: This should have a minimal impact on users as the flush API is only required for the post data API,
+    which was deprecated since 7.11.0.