In the first scene of the documentary “ RBG,” Gloria Steinem calls Supreme Court Justice Ruth Bader Ginsburg “a superhero.” The label could not be more appropriate — or more stifling. In recent years, Ginsburg, 85, has acquired a cultlike following.You can buy baby onesies and stud earrings of the jabot she wears when she delivers dissents; her workout routine has become a source of fitspiration; and, most recently, Felicity Jones played a young Ginsburg in the movie “On the Basis of Sex.” But, at the same time, her fragility has become clearer. Since Ginsburg’s appointment to the Supreme Court,...