The nature of the medium of documentary is built around an unknown. Filmmakers journey into a subject or a life without all the answers (the worst documentaries purport to hold all the cards), and what often makes a good documentary riveting is the narrative of exploration, the uncovering of a truth. But, what often defines the best documentaries is their ability to pivot and spot, mid narration, a wellspring of truth more compelling than that of the original focal point of the film. In Luke Korem’s new documentary “Dealt,” that’s just what happens. What is, at its outset, a rather charming film about the world-famous card mechanic Richard Turner and the obstacles he has overcome, winds up being a searching study of the painful nature of coming to terms with your own vulnerabilities.