Taken from http://www.slac.stanford.edu/comp/unix/package/rtems/doc/html/libm/libm.info.copysign.html i'd suggest to extend

  Return a float with the magnitude of x

to

  Return a float with the magnitude (absolute value) of x

It could probably help people less math-savvy in understand what's going to happen :)

Maybe also (only in rest doc) might be nice to describe what happens in case the arguments are NaN, f.e.:

>>> import math
>>> x = float('nan')
>>> math.copysign(1., x)
1.0
>>> math.copysign(-1., x)
1.0
>>> math.copysign(x, -1)
nan
>>> math.copysign(x, x)
nan

umedoblock: would you like to expand the patch with these notes (unless someone objects :)).