message: Add a note to JPEG encode_image
  The note is meant to explain why the method is not universally added to
  all other encoders, and the reasoning behind adding it specifically for
  this encoder.
diff: @@ -473,7 +473,15 @@ impl<'a, W: Write> JPEGEncoder<'a, W> {
  }
  }
  - /// Encodes the given image
  + /// Encodes the given image.
  + ///
  + /// As a special feature this does not require the whole image to be present in memory at the
  + /// same time such that it may be computed on the fly, which is why this method exists on this
  + /// encoder but not on others. Instead the encoder will iterate over 8-by-8 blocks of pixels at
  + /// a time, inspecting each pixel exactly once. You can rely on this behaviour when calling
  + /// this method.
  + ///
  + /// The Image in encoded with subsampling ratio 4:2:2
  pub fn encode_image<I: GenericImageView>(
  &mut self,
  image: &I,
  