os: clarify Rename docs for renaming to a directory

Fixes #68690

Change-Id: Iebc27d5f385d787e568550a863f59f16c7a79fd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/602178


Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Rob Pike <r@golang.org>

src/os/file.go

@@ -404,6 +404,7 @@ var lstat = Lstat
 
 // Rename renames (moves) oldpath to newpath.
 // If newpath already exists and is not a directory, Rename replaces it.
+// If newpath already exists and is a directory, Rename returns an error.
 // 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.