[NET]: Improve netdev->hard_start_xmit() documentation.

With improvements by Francois Romieu and David
S. Miller.
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/netdevices.txt

@@ -43,8 +43,21 @@ dev->get_stats:
 
 dev->hard_start_xmit:
 	Synchronization: dev->xmit_lock spinlock.
+	When the driver sets NETIF_F_LLTX in dev->features this will be
+	called without holding xmit_lock. In this case the driver 
+	has to lock by itself when needed. It is recommended to use a try lock
+	for this and return -1 when the spin lock fails. 
+	The locking there should also properly protect against 
+	set_multicast_list
 	Context: BHs disabled
 	Notes: netif_queue_stopped() is guaranteed false
+	Return codes: 
+	o NETDEV_TX_OK everything ok. 
+	o NETDEV_TX_BUSY Cannot transmit packet, try later 
+	  Usually a bug, means queue start/stop flow control is broken in
+	  the driver. Note: the driver must NOT put the skb in its DMA ring.
+	o NETDEV_TX_LOCKED Locking failed, please retry quickly.
+	  Only valid when NETIF_F_LLTX is set.
 
 dev->tx_timeout:
 	Synchronization: dev->xmit_lock spinlock.