docs: net: document netdev notifier expectations

We don't have a consistent state yet, but document where we think
we are and where we wanna be.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250401163452.622454-8-sdf@fomichev.me


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