math: special cases for Pow10; delete BUG

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/5477046

src/pkg/math/const.go

@@ -49,5 +49,3 @@ const (
 	MaxUint32 = 1<<32 - 1
 	MaxUint64 = 1<<64 - 1
 )
-
-// BUG(rsc): The manual should define the special cases for all of these functions.

src/pkg/math/pow10.go

@@ -9,6 +9,10 @@ package math
 var pow10tab [70]float64
 
 // Pow10 returns 10**e, the base-10 exponential of e.
+//
+// Special cases are:
+//	Pow10(e) = +Inf for e > 309
+//	Pow10(e) = 0 for e < -324
 func Pow10(e int) float64 {
 	if e <= -325 {
 		return 0