In a genre as towering and vast as American rhythm and blues, even the cultiest cult figures can feel larger than life. Such is the case with Jerry “Swamp Dogg” Williams, a 69-year-old Virginia native whose sprawling discography of rowdy outsider-soul has made him one of R&B’s most unsung journeymen — and perhaps one of the most self-effacing, too. “Some people have used the term ‘ahead of his time,’ ” Williams rasps over the phone from his California home.