"Currently in +1 cities" means that there is one city that would get more unhappy population if you lose the resources. For example, if you have four already unhappy cities, you would see "currently in +1 cities" when trading away your second instance of a resource that affects three cities. But also, if you have three happy cities but one of them is at exactly equal happiness and unhappiness, you would see the same when trading away the last instance of a resource. All of this is aware of the order of which cities would lose access to a resource. Likewise "for +2 cities" means that there are two unhappy cities that would gain access to the resource and therefore have less unhappy population. I added this because I found it very intransparent in the diplomacy window if you would gain/lose anything from additional/fewer resources. Note that +0 doesn't necessarily mean that its useless. The second instance of a resource might not have any benefit because the affected cities are already happy/healthy, but the third might. Still, this is a lot more informative than what we previously had.