You can't add comments to cards controlled by a hard form, but you can include a commentable card on a form. For example the +discussion card below appears because "+discussion" is included on Feature+*type+*content. (In open/closed views — again, comments on included cards only work if they're open).
If things sour in the conversation somehow and you get into some sort of who-said-what battle, you can always go back through the card's revision history (see its Changes" tab).
comments+Tips