Chris Rock’s documentary “Good Hair” hits theatres nationwide today. In the movie—which was first shown at the Sundance Film Festival this past winter and has been available in select cities since early October—Mr. Rock uses his wit to explore the “$9 billion” business of black hair by visiting salons, labs and hair shows and interviewing a bevy of celebrities about the struggles they have had with their hair. The documentary shines a light on the wide range of services and products Afri...