docs: netdev: turn the net-next closed into a Warning

Use the sphinx Warning box to make the net-next being closed
stand out more.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>