os: document that Rename is not atomic on non-Unix platforms

Windows provides no reliable way to rename files atomically.

The Plan 9 implementation of os.Rename performs a deletion
if the target exists.

Change-Id: Ife5f9c97b21f48c11e300cd76d8c7f715db09fd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/462395
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: adonovan

src/os/file.go

@@ -338,6 +338,7 @@ var lstat = Lstat
 // Rename renames (moves) oldpath to newpath.
 // If newpath already exists and is not a directory, Rename replaces it.
 // OS-specific restrictions may apply when oldpath and newpath are in different directories.
+// Even within the same directory, on non-Unix platforms Rename is not an atomic operation.
 // If there is an error, it will be of type *LinkError.
 func Rename(oldpath, newpath string) error {
 	return rename(oldpath, newpath)