Documentation: devres: note checking needs when converting

There are a number of cases where conversions to devm_* API have been
done but developers forgot that this conversion may imply that return
values need to be checked for failure of internal resource handling
like allocation. While this should be clear, it does seem to be a
relatively common issue with API conversions or maybe "managed" is
being misunderstood ? So add a note to make the scope of "managed" clear.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>