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 <adonovan@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Alan Donovan <adonovan@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>