Add missing issue sorting option relative_position_desc (or improve documentation)

Summary

Right now if API user tries to list issues sorted by relative_position, in descending order (that's reverse order to the one in Boards, so the Board-bottom issues should be listed first), Gitlab will silently ignore the sorting.

See Issue.simple_sorts, where that sorting option is missing.

I'm not sure whether it's a bug or a design decision, but there's nothing in the documentation that states this behavior should be expected, and it confused me a lot (as the descending order is the default one).

Steps to reproduce

1. Make a project.
2. Add some issues.
3. Go to Boards, change ordering.
4. Try GETting http://127.0.0.1/api/v4/projects/1/issues?order_by=relative_position&sort=asc, it will give you issues in the same order as they appear in Boards.
5. Try GETting http://127.0.0.1/api/v4/projects/1/issues?order_by=relative_position&sort=desc or http://127.0.0.1:3000/api/v4/projects/1/issues?order_by=relative_position, it will give you issues sorted by their ID.

What is the expected correct behavior?

sort=desc should give a reverse of whatever sort=asc gives.

Gitlab version

# fragment of docker-compose.yml
  gitlab:
    image: gitlab/gitlab-ce:13.2.6-ce.0
    volumes:
      - ~/daryush-dev-gitlab/config:/etc/gitlab
      - ~/daryush-dev-gitlab/logs:/var/log/gitlab
      - ~/daryush-dev-gitlab/data:/var/opt/gitlab
    ports:
      - 80:80

This bug is still present on master (code linked above). Documentation I was reading: https://docs.gitlab.com/ee/api/issues.html