Add known issue with offline agents failing to unenroll from Fleet

docs/en/ingest-management/release-notes/release-notes-8.5.asciidoc

@@ -52,6 +52,43 @@ This issue will be resolved in version 8.5.2. We advise Windows users not to
 upgrade to version 8.5.1.
 ====
 
+[[known-issue-2383]]
+.Offline {agent}s fail to unenroll after timeout has expired
+[%collapsible]
+====
+
+*Details*
+
+A {fleet-server-issue}/2091[known issue] in {fleet-server} 8.5.1 prevents
+offline agents from being automatically unenrolled after the unenrollment
+timeout expires.
+
+*Impact* +
+
+Offline agents will be displayed in the {fleet} Agents list until you explicitly
+force <<unenroll-elastic-agent,unenroll>> them. You can do this through the
+{fleet} UI or by using the API.
+
+To use the API:
+
+. Find agent's ID. Go to *{fleet} > Agents* and click the agent to see its
+details. Copy the Agent ID.
+
+. In a terminal window, run:
++
+[source,shell]
+----
+curl -u <username>:<password> --request POST \
+  --url <kibana_url>/api/fleet/agents/<agentID>/unenroll \
+  --header 'content-type: application/json' \
+  --header 'kbn-xsrf: xx' \
+  --data-raw '{"force":true,"revoke":true}' \
+  --compressed
+----
++
+Where `<agentID>` is the ID you copied in the previous step.
+
+====
 
 [discrete]
 [[enhancements-8.5.1]]