Documentation: netdev-FAQ: suggest how to post co-dependent series

Make an explicit suggestion how to post user space side of kernel
patches to avoid reposts when patchwork groups the wrong patches.

v2: mention the cases unlike iproute2 explicitly

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>