docs: netdev: document guidance on cleanup patches

The purpose of this section is to document what is the current practice
regarding clean-up patches which address checkpatch warnings and similar
problems. I feel there is a value in having this documented so others
can easily refer to it.

Clearly this topic is subjective. And to some extent the current
practice discourages a wider range of patches than is described here.
But I feel it is best to start somewhere, with the most well established
part of the current practice.

Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241009-doc-mc-clean-v2-1-e637b665fa81@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>