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.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>