math/big: simplify GCD docs
    
    We don't usually document past behavior (like "As of Go 1.14 ...") and
    in isolation the current docs made it sound like a and b could only be
    negative or zero.
    
    Change-Id: I0d3c2b8579a9c01159ce528a3128b1478e99042a
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217302
    Reviewed-by: Ian Lance Taylor <iant@golang.org>