New York:  Few billionaires are willing to reveal much about their taxes. But the Facebook co-founder Mark Zuckerberg has indirectly done so in the documents for his company’s public stock offering later this year. Mr. Zuckerberg, 27, one of the world’s youngest billionaires, plans to exercise stock options with an estimated value of $5 billion ahead of the offering. That could create a stunning tax bill of $2 billion. Although…