Jean Shepherd is onstage in Rochester, biting the hand that feeds him. And the hand, characteristically, is eating it up. "Every night you watch public television," Shepherd is telling the audience of public TV patrons. "A basic form of true masochism. On all the other channels they're whoopin' it up - they've got shows that take place in Hawaii; Tom Selleck is running on the beach with God-knows-who wearing a bikini - and you're watching a three-part documentary on the fishing industry. You're strong people." . . .