bonding: update documentation section after dev->trans_start removal

Drivers that use LLTX need to update trans_start of the netdev_queue.
(Most drivers don't use LLTX; stack does this update if .ndo_start_xmit
 returned TX_OK).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>