Add a warning about iid vs id to epic notes api

doc/api/notes.md

@@ -453,6 +453,11 @@ curl --request DELETE --header "PRIVATE-TOKEN: <your_access_token>" "https://git
 
 Gets a list of all notes for a single epic. Epic notes are comments users can post to an epic.
 
+NOTE:
+The epics notes API uses the epic ID instead of epic IID. If you use the epic's IID, GitLab returns either a 404
+error or notes for the wrong epic. It's different from the [issue notes API](#issues) and
+[merge requests notes API](#merge-requests).
+
 ```plaintext
 GET /groups/:id/epics/:epic_id/notes
 GET /groups/:id/epics/:epic_id/notes?sort=asc&order_by=updated_at