Warren Buffett, the billionaire investor known as much for his folksy wisdom as his investing prowess, announced Tuesday that he has been diagnosed with cancer. In a letter to shareholders, Buffett, 81, said that he had been diagnosed with stage I prostate cancer. “The good news is that I've been told by my doctors that my condition is not remotely life-threatening or even debilitating in any meaningful way,” Buffett said in the letter. Buffett said that he will begin a two-month treatment of...