If 'CONFIG_KUNIT=m', letting kunit tests that do not support loadable
module build depends on 'KUNIT' instead of 'KUNIT=y' result in compile
errors.  This commit updates the document for this.

Fixes: 9fe124bf ("kunit: allow kunit to be loaded as a module")
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>