I will forward your suggestion to the documentation team.
The exact steps for inventory control will change based on the gateway used.
The general gist is to add code after the transaction is passed to gateway that will loop through the cart and for each line item in the cart, perform an updated using the cart ID column to tell what record to update and the quantity column as the inventory subtraction amount.
If you need help implementing this, we can help you in a premiere support appointment:
http://www.webassist.com/premier_request.php