message: clarify safety comments in `Drop` impl
diff: @@ -202,6 +202,7 @@ struct OptionCell<T> {
  impl<T> Drop for OptionCell<T> {
  fn drop(&mut self) {
  if self.is_init {
  + // Safety: `value` is guaranteed to be fully initialized when `is_init` is true.
  // Safety: The cell is being dropped, so it can't be accessed again.
  drop(unsafe { self.value.read() });
  }
  