This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
If the image has a description, description is taken for the back of the image.
The description is plain text only.
If you like to take rich text for the back, you can take the bodytext of the content element “Image Rotate”.
Description of the image must be empty (see section above).