Put prompt metadata into images' EXIF::Description, not EXIF::UserComment

There's no implied encoding standard for UserComment even with the UNICODE indicator and most image metadata previewers do not assume it's UTF-16LE, showing what looks like Chinese (because they're assuming BE byte order).

Put the prompt metadata into Description as UTF-8, where OSes like MacOS and Windows will actually index it and/or show it in file properties. UserComment was a nice idea in the early days of TIFF but it was never clear about implementation details; ExifTool is only able to parse that field because it manually analyzes the stream to guess encoding and endianness.