docs: netdev: broaden the new vs old code formatting guidelines

Convert the "should I use new or old comment formatting" to cover
all formatting. This makes the question itself shorter.

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