runtime/debug: document that Stack is deprecated

Fixes #4070.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7004050

src/pkg/runtime/debug/stack.go

@@ -29,6 +29,8 @@ func PrintStack() {
 // For each routine, it includes the source line information and PC value,
 // then attempts to discover, for Go functions, the calling function or
 // method and the text of the line containing the invocation.
+//
+// This function is deprecated. Use package runtime's Stack instead.
 func Stack() []byte {
 	return stack()
 }