In “Jimmy Scott: If You Only Knew,” the septuagenarian jazz singer with a distinctive high-pitched voice looks back on his long road to stardom: Scott didn't hit it big until he was in his 60s, and while his success is tied to the pitch of his voice, he got that pitch (though not his talent) because of a hormonal disorder that struck him at age 12. Later would come bad marriages and worse record contracts, but the sunny Scott, now 78, recalls it all with the equanimity of a survivor.