better cancel condition for findroot(); better docstring
better cancel condition for findroot(); better docstring

error < tol*max(1, abs(x)) should theoretically work better for large x than
error < tol. However, this needs to be tested.

The new cancel condition does not seem to have unwanted side effects.


fredrik-johansson/mpmath,klkuhlm/mpmath,JensGrabner/mpmath,bjodah/mpmath,bjodah/mpmath,JensGrabner/mpmath,klkuhlm/mpmath