[Docs] `gitlab_shell_ssh_port` (Docker)

When this GitLab SSH proxy is working, the `gitlab_shell_ssh_port` setting has to be removed (when previously used for a custom SSH port), otherwise GitLab will still add the non-default SSH port to SSH URLs on web frontend (notably the SSH git clone command in a repository).
See https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/1767#note_53544860.