math: improve documentation of Copysign

Name the arguments in a way that is more self-describing.
Many code editor tools show a snippet of the function and
its arguments. However, "x" and "y" are not helpful in determining
which is the sign and which is the magnitude,
short of reading the documentation itself.

Name the sign argument as "sign" to be explicit.
This follows the same naming convention as IsInf.

Change-Id: Ie3055009e475f96c92d5ea7bfe9828eed908c78b
Reviewed-on: https://go-review.googlesource.com/c/go/+/400177
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: dsnet

src/math/copysign.go

@@ -4,9 +4,9 @@
 
 package math
 
-// Copysign returns a value with the magnitude
-// of x and the sign of y.
-func Copysign(x, y float64) float64 {
-	const sign = 1 << 63
-	return Float64frombits(Float64bits(x)&^sign | Float64bits(y)&sign)
+// Copysign returns a value with the magnitude of f
+// and the sign of sign.
+func Copysign(f, sign float64) float64 {
+	const signBit = 1 << 63
+	return Float64frombits(Float64bits(f)&^signBit | Float64bits(sign)&signBit)
 }