Add documentation on how cart transform api, product discount api and order discount api works together

[peter-visualsquares]

Since all those api can modify price, would like to know below

1. Running sequence of different api. For example does cart transform api run first?
2. Since they both interact with price, how does it works together. For example, if a cart transform api modify line item price. Then do produce discount see the updated price? Same question for product discount api and order discount api. Can order discount api see the price after all product discount apply?

Thanks!