It may just be the diaper of baby Rosamund Watson, whose arrival at the start of the fourth season of “Sherlock” disrupts the relationship between brilliant, demanding detective Sherlock Holmes and his long-suffering friend Dr. John Watson. The offspring of John and his enigmatic wife Mary – with Sherlock as a somewhat skeptical godfather – is one of the few things program-makers are willing to reveal before the first of three new episodes airs Jan. 1 on the BBC in Britain and on PBS in the United States.