docs: netdev: document guidance on cleanup.h

docs: netdev: document guidance on cleanup.h

Document what was discussed multiple times on list and various
virtual / in-person conversations. guard() being okay in functions
<= 20 LoC is a bit of my own invention. If the function is trivial
it should be fine, but feel free to disagree :)

We'll obviously revisit this guidance as time passes and we and other
subsystems get more experience.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20240830171443.3532077-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>