Documentation: kunit: Warn that exit functions run even if init fails

KUnit's exit functions will run even if the corresponding init function
fails. It's easy, when writing an exit function, to assume the init
function succeeded, and (for example) access uninitialised memory or
dereference NULL pointers.

Note that this case exists and should be handled in the documentation.

Suggested-by: Benjamin Berg <benjamin@sipsolutions.net>
Link: https://lore.kernel.org/linux-kselftest/a39af0400abedb2e9b31d84c37551cecc3eed0e1.camel@sipsolutions.net/


Reviewed-by: Sadiya Kazi <sadiyakazi@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>