Workflow very unclear for confidential merge requests

After creating a confidential merge request (eg. a merge request created on a private fork of the module we're trying to fix an issue for), it's very unclear what it is needed to merge the original repository.

The flow for an issue I recently tried to deal with in the GNOME GitLab went something like:

• go to confidential issue
• read instructions in "create confidential merge request" drop-down
• fork repository in my own namespace
• make repository private (via sysadmin, see #334155)
• come back to issue, and click "create confidential merge request" button
• empty branch and MR are created in my private repo

and here, there's no explanations as to how to get the changes one might push to this branch back to the original repository. I could not change the target of the branch to something that would target the original repository, and there were no instructions on how to get the changes merged to the real repo.

https://gitlab.gnome.org/help/user/project/merge_requests/index.md didn't contain any specific information either.