OpenAPI: document getting a specific nutrient field

[alexgarel]

In API.pm, customize_response_for_product

        # Apps can request the full nutriments hash
        # or specific nutrients:
        # - saturated-fat_prepared_100g : return field at top level
        # - nutrients|nutriments.sugars_serving : return field in nutrients / nutriments hash
        elsif ($field =~ /^((nutrients|nutriments)\.)?((.*)_(100g|serving))$/) {

So those works:

• https://world.openfoodfacts.org/api/v2/search?code=3263859883713,8437011606013,6111069000451&fields=code,product_name,nutriments.energy_100g
• https://world.openfoodfacts.org/api/v2/search?code=3263859883713,8437011606013,6111069000451&fields=code,product_name,energy_100g

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