[docs] Documentation for math.atan2() can be improved.

Hullo, The documentation for math.atan2( y, x ) says: --------8<-------- The point of atan2() is that the signs of both inputs are known to it, so it can compute the correct quadrant for the angle. --------8<-------- This is correct, but incomplete. Another big advantage of using atan2() is that it works perfectly in the regime where x, the denominator, is close to, or equal to zero. While atan() will fail for x == 0, and may give inaccurate answers for x~= 0, atan2() should work fine. Thanks, Niraj