I realize this is a super old post, but I thought someone might run across it and find this helpful. Document Approvals and Proof Approvals are not on the list of what can be filtered in via a custom section of an object:
Display dashboards in the left panel of objects
For more information about adding a dashboard under an object, see the section Add a dashboard in the left panel of a Workfront object or area in this article.
When you add a dashboard to a custom section under an object, the object acts as a filter for the dashboard. For example, if you add a task report on a dashboard and you add the dashboard to a project, the custom section that contains the dashboard on the project displays only tasks on the project you are viewing.
The following objects are filtered for the object under which they display, if that object is higher in hierarchy than them:
- Project
- Task
- Issue
- Approval Process
- Note
- Document