I have to admit I was a little nervous when I first heard “South Park” was airing an episode about medical marijuana this week. The show’s creators have spent the last 13 years mocking public policy issues both credible and contemptible, and—as much as I try to have a sense of humor about these things—I was legitimately worried that the same guys who recently referred to Sarah Jessica Parker as a “transvestite donkey witch” might not portray medical marijuana in the best possible light. Luckily,…