Better document imgid and id

[teolemon]

What

• Better document the difference between imgid and id
• you can find imgid in the request, it's a unique id assigned to uploaded images, regardless of whether they are selected or not.
• You apply a rotation to a raw image id, and you place it in id like front_fr (which is a selection)
  https://world.openfoodfacts.org/api/v0/product/3017620422003.json?fields=images

https://world.openfoodfacts.org/product/3760284870115/tiramisu-patissier
https://world.openfoodfacts.org/images/products/376/028/487/0115/1.jpg (1 is imgid)
https://world.openfoodfacts.org/images/products/376/028/487/0115/front_fr.8.full.jpg (front_fr is id, the 8 gets assigned automatically)

[teolemon]

https://openfoodfacts.github.io/openfoodfacts-server/api/ref-v2/#cmp--parameters-imgid