docs: netdev: add a note on selftest posting

docs: netdev: add a note on selftest posting

We haven't had much discussion on the list about this, but
a handful of people have been confused about rules on
posting selftests for fixes, lately. I tend to post fixes
with their respective selftests in the same series.
There are tradeoffs around size of the net tree and conflicts
but so far it hasn't been a major issue.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Link: https://patch.msgid.link/20250306180533.1864075-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>