net: sysfs: add documentation entries for /sys/class/<iface>/queues

Add sysfs documentation for the various attributes of a network
interface exposed in /sys/class/<iface>/queues/{rx,tx}-<queue>/

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>