FMLA forms and documentation should never be put in the personnel record. Our office files FMLA forms in the employee's medical record which are kept in a separate locked cabinet away from the personnel files since they contain PHI.
I honestly don't believe that putting them altogether in one binder is a good idea either. If an authorized person is looking for one specific employee's FMLA forms, there is potential to reveal unauthorized access to PHI on other employees that could open the door for trouble.