message: Add comment for the flag diff check
diff: @@ -107,6 +107,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  if let Some(FileHandle { flag: old_flag, .. }) =
  this.machine.file_handler.handles.get_mut(&fd)
  {
  + // Check that the only difference between the old flag and the current flag is
  + // exactly the `FD_CLOEXEC` value.
  if flag ^ *old_flag == fd_cloexec {
  *old_flag = flag;
  } else {
  