Ken Burns’s new documentary on World War II begins airing on public television next weekend, and if you haven’t heard about it, you’re probably alone. “The War” is being rolled out with a promotional campaign unlike any PBS has ever mounted before, and for good reason. “The world of broadcasting has changed enormously over the last 17 years. So has the world of documentaries.” What hasn’t changed is the trademark Burns style, and PBS is counting on the combination of quality and familiarity to deliver a ratings bonanza.