math: make function documentation more regular

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/5994043
diff --git a/src/pkg/math/atanh.go b/src/pkg/math/atanh.go
index 5b5d468..113d5c1 100644
--- a/src/pkg/math/atanh.go
+++ b/src/pkg/math/atanh.go
@@ -36,7 +36,7 @@
 //	atanh(+-1) is +-INF with signal.
 //
 
-// Atanh(x) calculates the inverse hyperbolic tangent of x.
+// Atanh returns the inverse hyperbolic tangent of x.
 //
 // Special cases are:
 //	Atanh(1) = +Inf