Update docs for epic deletion permissions

Problem

This communicates that Planner can create any epic. In reality, Planner can only delete epics they authored.

Also noted here (https://docs.gitlab.com/user/permissions/#project-planning) is that any role can delete tasks that they've authored, including Guest.

All work item types should theoretically be using the same exact set of permissons at this point.

Proposal

• Get clarification from engineering on what the permissions are and ensure they are consistent across all work item types.
• Improve docs to accurately reflect the current permissions in the product.