kunit: fix struct kunit_attr header

Add parameter descriptions to struct kunit_attr header for the
parameters attr_default and print.

Fixes: 39e92cb1

 ("kunit: Add test attributes API structure")

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308180127.VD7YRPGa-lkp@intel.com/

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