docs: networking: add guidance on what to push via extack

Every now and then someone tries to duplicated extack
messages to dmesg. Document our guidance against this.
Also indicate that system level faults should continue
to go to system logs. The high level thinking is to try
to distinguish between what's important to the user vs
system admin.

Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20260611172149.1877704-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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