With auth on, you can no longer manipulate the system.users collection directly in 2.5.2, e.g.

db.system.users.insert({...})

I'm not sure if the new commands exist yet specified in https://jira.mongodb.org/browse/SERVER-6246.

If they are not, then the release notes should point to .addUser and removeUser helpers. If they do work, then they should be documented.