doc:networking: Update comment for dev_id field in netdevice.h

This patch updates the comment for 'dev_id' field in
'include/linux/netdevice.h' to reflect the intended
usage of 'dev_id'.

References: http://marc.info/?l=linux-netdev&m=136992115300526&w=2
References: http://marc.info/?l=linux-netdev&m=137062569014612&w=2

Signed-off-by: Narendra K <narendra_k@dell.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1174,8 +1174,10 @@ struct net_device {
 	unsigned char		addr_assign_type; /* hw address assignment type */
 	unsigned char		addr_len;	/* hardware address length	*/
 	unsigned char		neigh_priv_len;
-	unsigned short          dev_id;		/* for shared network cards */
-
+	unsigned short          dev_id;		/* Used to differentiate devices
+						 * that share the same link
+						 * layer address
+						 */
 	spinlock_t		addr_list_lock;
 	struct netdev_hw_addr_list	uc;	/* Unicast mac addresses */
 	struct netdev_hw_addr_list	mc;	/* Multicast mac addresses */