os: improve documentation on Rename

Rename should document that it returns *LinkError,
like Create and Stat do.

Fixes #10061

Change-Id: I7bfe8b0267f6c4a57dd6b26cba44928714711724
Reviewed-on: https://go-review.googlesource.com/12353Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -259,6 +259,7 @@ func Create(name string) (file *File, err error) {
 var lstat = Lstat
 
 // Rename renames (moves) a file. OS-specific restrictions might apply.
+// If there is an error, it will be of type *LinkError.
 func Rename(oldpath, newpath string) error {
 	return rename(oldpath, newpath)
 }