math: the trig functions work in radians; document that

Fixes #6543
6543 is also a fine NGC object.

R=golang-dev, dsymonds, kamil.kisiel, minux.ma
CC=golang-dev
https://golang.org/cl/14515044

src/pkg/math/asin.go

@@ -11,7 +11,7 @@ package math
 	after appropriate range reduction.
 */
 
-// Asin returns the arcsine of x.
+// Asin returns the arcsine, in radians, of x.
 //
 // Special cases are:
 //	Asin(±0) = ±0
@@ -44,7 +44,7 @@ func asin(x float64) float64 {
 	return temp
 }
 
-// Acos returns the arccosine of x.
+// Acos returns the arccosine, in radians, of x.
 //
 // Special case is:
 //	Acos(x) = NaN if x < -1 or x > 1

src/pkg/math/atan.go

@@ -87,7 +87,7 @@ func satan(x float64) float64 {
 	return Pi/4 + xatan((x-1)/(x+1)) + 0.5*Morebits
 }
 
-// Atan returns the arctangent of x.
+// Atan returns the arctangent, in radians, of x.
 //
 // Special cases are:
 //      Atan(±0) = ±0

src/pkg/math/sin.go

@@ -109,7 +109,7 @@ var _cos = [...]float64{
 	4.16666666666665929218E-2,   // 0x3fa555555555554b
 }
 
-// Cos returns the cosine of x.
+// Cos returns the cosine of the radian argument x.
 //
 // Special cases are:
 //	Cos(±Inf) = NaN
@@ -165,7 +165,7 @@ func cos(x float64) float64 {
 	return y
 }
 
-// Sin returns the sine of x.
+// Sin returns the sine of the radian argument x.
 //
 // Special cases are:
 //	Sin(±0) = ±0

src/pkg/math/tan.go

@@ -73,7 +73,7 @@ var _tanQ = [...]float64{
 	-5.38695755929454629881E7, //0xc189afe03cbe5a31
 }
 
-// Tan returns the tangent of x.
+// Tan returns the tangent of the radian argument x.
 //
 // Special cases are:
 //	Tan(±0) = ±0