Documentation updates for network interfaces.
1. Add doc for netif_napi_add
2. Remove doc for unused returns from netif_rx
3. Add doc for netif_receive_skb
[ Incorporated minor mods from Randy Dunlap -DaveM ]
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>