[D9] Allow adding descriptions to user roles (and show them in the roles listing page)

This is the same as https://www.drupal.org/project/drupal/issues/256287:

It would be useful to be able to document on the site why a certain role exists.

The feature is currently provided by the contrib module https://github.com/backdrop-contrib/role_help (that module also allows for a separate help text field that is shown to the user, but I don't think that this should be provided by core).

Advocate: @quicksketch