How (and when) do you describe "affordance" in your Interaction Design?

Take the following example of an animation in a webstore.

Selecting a whine will make it float and it will fly to your "bag" when clicked "add to cart"

Say you have to write an Interaction Design document about this. I would imagine it would look something like this (VERY simplified of course):


User-story

"As a customer, I can add a wine bottle to my cart, so I can buy it"

Use-case:

  1. Move the mouse over the wine bottle
  2. Click the "Add to Cart" button
  3. Result: Wine bottle is now in the "My bag" section

Mock-up

Basic mock-up


But where/when do I describe the fact that the bottle will float when the mouse hovers over it, and visually moves to the cart box when "add to cart" is clicked? I don't see it as part of the Story/Case/Mock-up at all.