doc: announce ABI change for PCI class

doc: announce ABI change for PCI class

The purpose of this patch is used to add a new field
"class" in rte_pci_id structure. The new class field includes
class_id, subcalss_id, programming interface of a pci device.
With this field, we can identify pci device by its class info,
which can be more flexible instead of probing the device by
vendor_id OR device_id OR subvendor_id OR subdevice_id.
For example, we can probe all nvme devices by class field, which
can be quite convenient.

Signed-off-by: Ziye Yang <ziye.yang@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Cunming Liang <cunming.liang@intel.com>