A private four-day symposium on pedophilia was held at the Vatican last week.  Senior academics and therapists from the United States, Canada and Germany were invited.  Vatican officials released little information about the symposium, saying details would be published in a few months.  Eight professors who were listed as participating included Dr. Martin P. Kafka of Harvard Medical School; Dr. Ron Langevin of the University of Toronto; Dr. William Marshall of Queens University in Kingston, Canada; Dr. Karl Hanson, a Canadian official; and four German specialists.