The songs that made her famous—“The Greatest Love of All,” “I Wanna Dance With Somebody (Who Loves Me),” “One Moment in Time”—tell you almost nothing about Whitney Houston, who, as Kevin Macdonald’s new documentary vividly catalogues, had more than her share of secrets. “Whitney” is not the first posthumous documentary about the singer, but it goes startlingly deep, both in the damage it dredges up and in the context it gives to her achievements.