doc, net: Update ndo_start_xmit return type and values

Commit dc1f8bf6 ('netdev: change
transmit to limited range type') changed the required return type and
9a1654ba

 ('net: Optimize
hard_start_xmit() return checking') changed the valid numerical
return values.

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