Document WriteToFile overwrite behavior, remove misplaced FIXME

mfer/output.go

@@ -5,9 +5,10 @@ import (
 	"os"
 )
 
+// WriteToFile writes the manifest to the given path.
+// It will overwrite any existing file. Callers should check for existing files
+// before calling if overwrite confirmation is needed.
 func (m *manifest) WriteToFile(path string) error {
-	// FIXME refuse to overwrite without -f if file exists
-
 	f, err := os.Create(path)
 	if err != nil {
 		return err