Imagine making a documentary about one of the 20th century’s leading opponents of the Ku Klux Klan -- without ever talking about the evil of the KKK itself. If that sounds like malpractice, consider PBS’s new documentary on the life of William F. Buckley Jr. “The Incomparable Mr. Buckley,” the latest installment in the “American Masters” series, has much to say about anti-Communism but never…