Choose Edit - Track Changes - Manage - List tab. Click an entry in the list and open the context menu. Choose Edit Comment.
You can attach a comment when the cursor is in a changed text passagethe changed cell is selected, or in the Manage Changes dialog.
Comments are displayed as callouts in the sheet when you rest your mouse pointer over a cell with a recorded change. You can also view comments that are attached to a changed cell in the changes list in the Manage Changes dialog.