[docs] [issue12211] Better document math.copysign behavior.

[Mark Dickinson]

Mark Dickinson added the comment:

> It seems to just copy the sign bit no matter what x and y are.

Yep, that's exactly what happens.  The sign bit from y just gets blindly transferred to x, with no regard to any meaning of x or y.  So it works exactly the same way for zeros, NaNs, infinities, or whatever.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12211>
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