Add info about which country is used for tax calculation in case of different addresses

[szczecha]

We are missing information in the docs on which country it takes to calculate taxes it can be misleading for checkout with digital products when customer pass the shipping address at checkoutCreate mutation.

The shipping address is always set when supplied in the input.

The order of resolving the checkout country code is always as follows:
Shipping address
Billing address
The channel's default country

https://www.notion.so/saleor/Tax-addresses-c831a50c95764e78a7e4a30798a2cc3d