How much can a supervisor inquire about an employee’s reason for FMLA leave?

Q: “Our Human Resources Department processes Family and Medical Leave Act paperwork and tracks FMLA use. When HR receives the completed Certification of Health Care Provider form, does the supervisor whom the employee reports to have the right to see the form and/or know the medical reason for the FMLA leave, or do they just need to know that the employee is covered under the FMLA and how long it is anticipated that they will be out?” – Ann, Illinois