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.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/process/maintainer-netdev.rst

@@ -375,6 +375,22 @@ When working in existing code which uses nonstandard formatting make
 your code follow the most recent guidelines, so that eventually all code
 in the domain of netdev is in the preferred format.
 
+Using device-managed and cleanup.h constructs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Netdev remains skeptical about promises of all "auto-cleanup" APIs,
+including even ``devm_`` helpers, historically. They are not the preferred
+style of implementation, merely an acceptable one.
+
+Use of ``guard()`` is discouraged within any function longer than 20 lines,
+``scoped_guard()`` is considered more readable. Using normal lock/unlock is
+still (weakly) preferred.
+
+Low level cleanup constructs (such as ``__free()``) can be used when building
+APIs and helpers, especially scoped iterators. However, direct use of
+``__free()`` within networking core and drivers is discouraged.
+Similar guidance applies to declaring variables mid-function.
+
 Resending after review
 ~~~~~~~~~~~~~~~~~~~~~~