BACKPORT: Documentation: kunit: Update Kconfig parts for KUNIT's module support

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.

Bug: 254441685
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>
(cherry picked from commit f0b62039)
[Lee: Omit the Documentation updates, since the relevant section is missing]
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: Ia7fac7696d7078a9b580c4814df77ac5d54bcfc9