math: fix gamma doc, link to OEIS

Fixes #2940.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5645078

src/pkg/math/const.go

@@ -6,7 +6,7 @@
 package math
 
 // Mathematical constants.
-// Reference: http://www.research.att.com/~njas/sequences/Axxxxxx
+// Reference: http://oeis.org/Axxxxxx
 const (
 	E   = 2.71828182845904523536028747135266249775724709369995957496696763 // A001113
 	Pi  = 3.14159265358979323846264338327950288419716939937510582097494459 // A000796

src/pkg/math/gamma.go

@@ -116,7 +116,7 @@ func stirling(x float64) float64 {
 //	Gamma(±Inf) = ±Inf
 //	Gamma(NaN) = NaN
 // Large values overflow to +Inf.
-// Negative integer values equal ±Inf.
+// Zero and negative integer arguments return ±Inf.
 func Gamma(x float64) float64 {
 	const Euler = 0.57721566490153286060651209008240243104215933593992 // A001620
 	// special cases