Documentation: kunit: Note that assertions should not be used in cleanup

As assertions abort the test cleanup process, they should be avoided
from within a suite's exit function, or from within resource 'free'
functions. Unlike with initialisation or main test execution, no further
cleanup will be performed after a failed assertion, potentially causing
a leak of resources.

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