Add instructions to purge Dependency Proxy cache from the rails console

doc/api/dependency_proxy.md

@@ -12,6 +12,9 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 Deletes the cached blobs for a group. This endpoint requires group admin access.
 
+CAUTION: **Warning:**
+[A bug exists](https://gitlab.com/gitlab-org/gitlab/-/issues/277161) for this API.
+
 ```plaintext
 DELETE /groups/:id/dependency_proxy/cache
 ```