This new Smithsonian TV documentary about the convicted murderer and composer of “Goodnight Irene” and other standards is televised the day before a new major boxed set, Lead Belly: The Smithsonian Folkways Collection, is released on CD. But it stands on its own, in no small part because it includes not only vintage clips, but newly recorded testimonials from such enthusiasts as Van Morrison. Many questions are answered, but here’s one of mine: I have no idea when people started spelling Huddie Ledbetter’s stage and recording name, Leadbelly, as two words. To me, it seems a bit arbi trary…