docs: kvm: vfio: Suggest KVM_DEV_VFIO_GROUP_ADD vs VFIO_GROUP_GET_DEVICE_FD ordering

as some vfio_device's open_device op requires kvm pointer and kvm pointer
set is part of GROUP_ADD.
Reviewed-by: NKevin Tian <kevin.tian@intel.com>
Signed-off-by: NYi Liu <yi.l.liu@intel.com>
Reviewed-by: NJason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230421053611.55839-1-yi.l.liu@intel.comSigned-off-by: NAlex Williamson <alex.williamson@redhat.com>

Documentation/virt/kvm/devices/vfio.rst

@@ -39,3 +39,8 @@ KVM_DEV_VFIO_GROUP attributes:
 	- @groupfd is a file descriptor for a VFIO group;
 	- @tablefd is a file descriptor for a TCE table allocated via
 	  KVM_CREATE_SPAPR_TCE.
+
+The GROUP_ADD operation above should be invoked prior to accessing the
+device file descriptor via VFIO_GROUP_GET_DEVICE_FD in order to support
+drivers which require a kvm pointer to be set in their .open_device()
+callback.