doc: announce removal of non-kernel based PCI probing

doc: announce removal of non-kernel based PCI probing

In order to optimize the DPDK PCI enumeration management, RTE_KDRV_NONE
based device driver probing will be removed in v20.08.
The legacy virtio is the only consumer of RTE_KDRV_NONE based
device  driver probe scheme.
The legacy virtio support will be available through existing VFIO/UIO
based kernel driver scheme.
More details at https://patches.dpdk.org/patch/69351/

Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>