Document the new product lists API

[teolemon]

Document the new product lists API
https://world.openfoodfacts.org/api/v2/search?code=8024884500403,3263855093192&fields=code,product_name

[CharlesNepote]

Including the max number of products to be requested. "I guess 1000 products won't be fine because of the largest header default limit of Nginx (8K). If you're requesting EAN13 barcodes the server should allow you 8000/14=571 products (I put 14 because you need to add a comma between each EAN). This limits can be modified, but I think we shouldn't."
https://openfoodfacts.slack.com/archives/C043X1X90/p1624606422044700