Do the same for ammo slot, i.e. add tooltip to show the acceptable ammo, just like this:
Missing such tooltip isn't a big problem for vanilla game since there isn't much ammo types. But this would definitely help modded games with more ammo types.
Edit: oh, I missed the ammo slots on the tool panel (the one at bottom left now). I imagine the acceptable ammos would show up for the ammo slots that have weapons above.