download_cover_art: Document entitytype

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>
pep8

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>
caa: Mention that the CAA will not return upscaled images

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>