net: ethool: Document struct ethtool_flow_ext

Add documentation for struct ethtool_flow_ext especially in regard
to what flags are needed for which fields.

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