os: clarify docs for Rename.

Three changes:
1. mention "move" to clarify things up.
2. use {old,new}path instead of {old,new}name, which makes it clear what
   relative path would do here.
3. mention "OS-specific restrictions might apply".

Fixes #6887.

R=golang-dev, alex.brainman, iant, r
CC=golang-dev
https://golang.org/cl/36930044

src/pkg/os/file.go

@@ -140,6 +140,9 @@ func (f *File) Write(b []byte) (n int, err error) {
 	if n < 0 {
 		n = 0
 	}
+	if n != len(b) {
+		err = io.ErrShortWrite
+	}
 
 	epipecheck(f, e)
 

src/pkg/os/file_plan9.go

@@ -313,8 +313,7 @@ func Remove(name string) error {
 	return nil
 }
 
-// Rename renames a file.
-func Rename(oldname, newname string) error {
+func rename(oldname, newname string) error {
 	var d syscall.Dir
 
 	d.Null()

src/pkg/os/file_posix.go

@@ -48,8 +48,7 @@ func Readlink(name string) (string, error) {
 	}
 }
 
-// Rename renames a file.
-func Rename(oldname, newname string) error {
+func rename(oldname, newname string) error {
 	e := syscall.Rename(oldname, newname)
 	if e != nil {
 		return &LinkError{"rename", oldname, newname, e}