net: core: document two new elements of struct net_device

As warned by "make htmldocs", there are two new struct elements
that aren't documented:

	../include/linux/netdevice.h:2159: warning: Function parameter or member 'unlink_list' not described in 'net_device'
	../include/linux/netdevice.h:2159: warning: Function parameter or member 'nested_level' not described in 'net_device'

Fixes: 1fc70edb ("net: core: add nested_level variable in net_device")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>