The only true Front of the Line courtesy extended by Disney is to "Make a Wish" program children. These are children with extremely serious (usually terminal) conditions.
Disney does not require Doctor Notes or diagosis to rent a wheelchair or scooter. In fact, requiring documentation would be illegal under federal ADA law.
The vast majority of wheelchair/
ECV guests do not fall into the Make A Wish category. Again, non-Make A Wish wheelchair and scooter guests do NOT get front of the line access, and wait through the normal queue lines. In the few older attractions where the queues are not retrofitted to accomodate wheelchairs, there is an alternative entrace, but wheelchair users generally wait as long, sometimes longer than ambulatory guests.