Improve QBluetoothTransferManager documentation

Change-Id: I918bd3b4134504d5bfc3e160adeb97187186df3d
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>