Fix documentation around Discount vs Specific Price

[markguinn]

The difference is confusing and not immediately clear. The normal user expectation seems to be that Discount will update the displayed price of the product. I think we should do two things:

1. In the short term update the documentation to spell out what each does.
2. In the longer term update the logic so Discounts affect the displayed price as well. That could be an option when the discount is created to maintain backwards compatibility.

Here's a sample explanation for the docs (taken from a recent gitter conversation): If you create a "Discount" from the Discounts tab in the CMS that is actually not applied until checkout. It becomes a line item in the order, just like if the customer typed in a coupon code but with the code. To discount the pricing for a specific product you would go in and edit that product and there is another set of discount controls in the Pricing tab on the product.