cmath: add package description

Also update range of Phase and Polar due to signed zero.
[Phase(cmplx(-1, +0)) = pi and Phase(cmplx(-1, -0)) = -pi]

R=rsc, r
CC=golang-dev
https://golang.org/cl/1235041

src/pkg/cmath/abs.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// The cmath package provides basic constants
+// and mathematical functions for complex numbers.
 package cmath
 
 import "math"

src/pkg/cmath/phase.go

@@ -7,5 +7,5 @@ package cmath
 import "math"
 
 // Phase returns the phase (also called the argument) of x.
-// The returned value is in the range (-Pi, Pi].
+// The returned value is in the range [-Pi, Pi].
 func Phase(x complex128) float64 { return math.Atan2(imag(x), real(x)) }

src/pkg/cmath/polar.go

@@ -6,7 +6,7 @@ package cmath
 
 // Polar returns the absolute value r and phase θ of x,
 // such that x = r * e**θi.
-// The phase is in the range (-Pi, Pi].
+// The phase is in the range [-Pi, Pi].
 func Polar(x complex128) (r, θ float64) {
 	return Abs(x), Phase(x)
 }