netdevice: document NETDEV_TX_BUSY deprecation.

This paraphrases DaveM (and steals some of his words) explaining why
a device shouldn't return NETDEV_TX_BUSY, even though it looks so inviting
to driver authors.

See http://www.spinics.net/lists/netdev/msg322350.html

Inspired-by: David Miller <davem@davemloft.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>