The details streaming from leaks and interview teasers in advance of Prince Harry’s memoir, “Spare,” have been sensational, even explosive. News organizations around the world have covered them breathlessly. There’s Harry’s description of his brother, William, knocking him to the floor during a fight, and Harry’s admission that he killed 25 people in Afghanistan. There’s also his claim that William and his wife, Kate, encouraged him to dress up as a Nazi, and his recollection of losing his virginity to an older woman in a field behind a bar. A tell-all by a member of the British royal family was...