Documentation: kunit: include example of a parameterized test

Commit fadb08e7 ("kunit: Support for Parameterized Testing")
introduced support but lacks documentation for how to use it.

This patch builds on commit 1f0e943d ("Documentation: kunit: provide
guidance for testing many inputs") to show a minimal example of the new
feature.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>