builtin: Changed documentation for rune type.

Fixes #4093.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/6536047

src/pkg/builtin/builtin.go

@@ -81,9 +81,8 @@ type uintptr uintptr
 // integer values.
 type byte byte
 
-// rune is an alias for int and is equivalent to int in all ways. It is
+// rune is an alias for int32 and is equivalent to int32 in all ways. It is
 // used, by convention, to distinguish character values from integer values.
-// In a future version of Go, it will change to an alias of int32.
 type rune rune
 
 // Type is here for the purposes of documentation only. It is a stand-in