It was a tragedy that touched upon several great national themes — the dislocation wrought by fading industry, the turmoil of immigration, the endurance of sports — and David C. Turnley, a Pulitzer Prize-winning photographer, was there to grapple with them all. He turned the story of the murder and how it shook the town into “Shenandoah,” a wrenching documentary that he shot, directed and produced.