Documentation: networking: Update netdev-FAQ regarding patches

Provide an explanation of what is expected with respect to sending new
versions of specific patches within a patch series, as well as what
happens if an earlier patch series accidentally gets merged).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>