Documentation: kunit: Add usage notes for kunit_add_action()

Add some basic documentation for kunit_add_action() and related
deferred action functions.

Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>