Where is the documentation for an official docker image?

This link, When to use Docker Official Images, says that These images have clear documentation. Where is it the documentation? For instance, for python image, I don’t see any documentation there.

It refers to the image description, which shows how to use the image.

The python image you picked has a description on how to create your own image using the python image as base image.

Other images like postgres will describe how to run the image as a container and explain the available environment variables and volumes (if existing).

The description will not include the documentation for the containerized application/service/programming language itself.