message: Add comment explaining use of Option<Decoder>
diff: @@ -28,6 +28,8 @@ where
  {
  dimensions: (u32, u32),
  color_type: ColorType,
  +
  + // We only use an Option here so we can call with_limits on the decoder without moving.
  inner: Option<tiff::decoder::Decoder<R>>,
  }
  