Owen Gleiberman Chief Film Critic There’s a moment in “Elton John: Never Too Late,” a robustly satisfying and emotional documentary about the life and career of Elton John, that captures him, in a most revealing way, in his ’70s heyday. It’s a clip from a television interview, in which Elton is explaining how he writes a song. The clip must be from 1971, and Elton, still looking like a puppy child, with rectangle-framed glasses and plenty of shaggy hair, sits at an upright piano and brings out a sheaf of lyrics — pages all written in longhand by his collaborator, Bernie Taupin.