As I watched this exchange, which is featured in “Avedon,” the new Netflix documentary directed by Ron Howard, it struck me that the photographer was being both honest and, perhaps, a little slippery about his work and its meaning. As one of the greatest portraitists of the twentieth century, up until his death, in 2004, Avedon was, indeed, omnivorous in his choice of subjects. He took pictures not just of authors and politicians but of actors, artists, activists, and socialites.