math/big: update comment on Int.GCD

Per the suggestion https://golang.org/cl/216200/2/doc/go1.14.html#423.

Updates #28878.

Change-Id: I654d2d114409624219a0041916f0a4030efc7573
Reviewed-on: https://go-review.googlesource.com/c/go/+/217104
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: griesemer

src/math/big/int.go

@@ -504,6 +504,8 @@ func (z *Int) Exp(x, y, m *Int) *Int {
 
 // GCD sets z to the greatest common divisor of a and b and returns z.
 // If x or y are not nil, GCD sets their value such that z = a*x + b*y.
+// As of Go 1.14, a and b may be zero or negative (before Go 1.14 both
+// a and b had to be > 0).
 // Regardless of the signs of a and b, z is always >= 0.
 // If a == b == 0, GCD sets z = x = y = 0.
 // If a == 0 and b != 0, GCD sets z = |b|, x = 0, y = sign(b) * 1.