Every Core API V1 response field is now described

Every field in every Core API V1 response now has a description, written from real API responses.

Also corrected to match the API:

• Retrieve orders returns order_line_id and product_id for each purchase. count is the number of orders per page, and a page can come back with fewer orders than count.
• Retrieve all products returns blacklisted for each product.
• Create order, Create mass orders and the product group calls now document the error responses they really return.