Directed by Bao Nguyen, the documentary, with Lionel Richie at its center (he’s our chief nostalgist and tour guide), is certainly “celebratory,” but it’s also honestly assembled and intensely pleasurable. No longtime fan of the song should miss it, but younger viewers may find themselves intoxicated as well. “The Greatest Night in Pop” pulls back the curtain on the perpetual smoke screen of music-god celebrity.