pkg: add missing godoc comments to windows versions

Mostly copied comments from unix files.

R=rsc
CC=golang-dev
https://golang.org/cl/5533057
diff --git a/src/pkg/os/sys_plan9.go b/src/pkg/os/sys_plan9.go
index 8ad89fb..07a7905 100644
--- a/src/pkg/os/sys_plan9.go
+++ b/src/pkg/os/sys_plan9.go
@@ -6,7 +6,7 @@
 
 package os
 
-func Hostname() (name string, err error) {
+func hostname() (name string, err error) {
 	f, err := Open("#c/sysname")
 	if err != nil {
 		return "", err