atomicfile: update docs to clarify behavior of argument
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// WriteFile writes data to filename+some suffix, then renames it
-// into filename.
+// into filename. The perm argument is ignored on Windows.
func WriteFile(filename string, data []byte, perm os.FileMode) (err error) {
f, err := ioutil.TempFile(filepath.Dir(filename), filepath.Base(filename)+".tmp")
if err != nil {
