docs: netdev: add note about Changes Requested and revising commit messages

One of the most commonly asked questions is "I answered all questions
and don't need to make any code changes, why was the patch not applied".
Document our time honored tradition of asking people to repost with
improved commit messages, to record the answers to reviewer questions.

Take this opportunity to also recommend a change log format.

Link: https://lore.kernel.org/r/20230322231202.265835-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>