Be fair of course in how you word it, but nothing speaks better than "I showed the new Admin X,Y,Z and he knows how to do X,Y,Z; here the signature to prove it". I know you are trying to avoid a new Admin coming in and then complaining about how the previous guy didn't know what the hell he was doing. Happens to everyone I'm afraid, but at least have your bases covered for what any replacement needs to know to operate in your permanent absence. It will also discourage the new admin from making any drama scenes with his/her new boss when he/she knows you have something in writing that is suppose to demonstrate/validate his/her new skills in the position. Other than that, don't burn any bridges, try to be helpful to the new Admin, when you have the free time, but don't go out your way and sacrifice your new job to help a struggling admin who might be in over his head due to fluffing up the resume.