I think the Comment field is only for internal use, personal notes about the expression, so to speak. I have yet to find a way to set up a hover/pop-up over expressions in a straight or pivot table. It is available in graph objects on the Presentation tab > "Popup as labels", however. And to add additional custom pop-up text, there's functionality for that (although buggy right now) on the Expression tab > "Text as pop-up". I haven't mastered that yet, however, but there are plenty of forums on the community for custom pop-ups I believe.
