I discovered in the Approval process that when you Approve or Reject in the Request itself, it doesn't provide a comment box, it just Approves or Rejects (and you'll have to comment in the Updates of the request). However, if the Approver goes to My Work (I assume Home is similar), and to Approvals, when they Approve or Reject with this method it DOES give them a comment box. See attached. So it's inconsistent and I've opened a ticket with Workfront, but I got the usual "by design" nonsense. But if you direct your users here, they can do it as they Reject (or Approve). Otherwise you'll have to tell them to update and tag the person in the Update tab.