If an approver decides to reject a process version or approve it conditionally he can always write a reason for it. It will be seen in the detailed approval results in the Approval history or in the Approval overview.
Since an approver is using Engage Process Viewer tool for process inspection he can also provide feedback comments there. These comments will be shown for a given processes together with the other comments made to its currently shared and published versions. An approval comment has its own 'approve/reject' icon 
on the comment's card.
To see only comments provided by approvers press the filter button on the Comments pane. A new line with filters will appear (press this button again to hide this line):
The three buttons on the right-hand side are used to filter on or off the comments made to the published, shared and being approved version of the process, respectively. Initially, all categories of comments are shown. Press the 'book' and 'share' buttons to filter out the other comments. Press them again to remove the filter.
A modeler can react on an approver comment. However, if it is a comment to a version that the approver has approved then the approver cannot read this reaction and cannot react on it because approved versions disappear from the list of processes offered for approval in Engage Process Viewer. Such reactions can only be seen in the Approval history.