docs: vfio: Add vfio device cdev description

This gives notes for userspace applications on device cdev usage.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Yanting Jiang <yanting.jiang@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230718135551.6592-27-yi.l.liu@intel.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>