net: netdevice.h: add a description for napi_defer_hard_irqs

Changeset 6f8b12d6 ("net: napi: add hard irqs deferral feature")
added a new element at struct net_device.

Add a description for it, based on what's described at the changeset
which added such feature.

Fixes: 6f8b12d6 ("net: napi: add hard irqs deferral feature")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/807a3840e7bc1562adefadb0535c9f47e6ab52e0.1592895969.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

include/linux/netdevice.h

@@ -1742,6 +1742,8 @@ enum netdev_priv_flags {
  *	@real_num_rx_queues: 	Number of RX queues currently active in device
  *	@xdp_prog:		XDP sockets filter program pointer
  *	@gro_flush_timeout:	timeout for GRO layer in NAPI
+ *	@napi_defer_hard_irqs:	If not zero, provides a counter that would
+ *				allow to avoid NIC hard IRQ, on busy queues.
  *
  *	@rx_handler:		handler for received packets
  *	@rx_handler_data: 	XXX: need comments on this one