[Documentation] Clarify Walmart "product_id" Parameter Documentation

[schaferyan]

The Walmart Product API requires a product_id parameter. However the value supplied doesn't need to be the product_id of the product, it can also be the us_item_id or upc. Our documentation states that it is better to use the us_item_id, but the product_id returns faster - which seems to be contradictory. In addition, it seems that the product page will include review results only if the us_item_id is used - we should mention this in our documentation.

The Walmart Reviews API also has a product_id parameter, but it only accepts the us_item_id, passing the product_id will not return results.

In both cases, the naming is already confusing, but I suspect there is nothing we can do about this since it's on Walmart's end. However we can be more explicit in our documentation and tooltips to help users not to be confused.

In the Walmart Product API documentation, we can clarify the benefits/drawbacks of supplying the product_id or the us_item_id as the value of the product_id parameter.

[ilyazub]

product_id will not return results.

It's a bug or something changed on Walmart's end. product_id supposed to accept all of upc, product_id, and us_item_id because Walmart responded with and accepted multiple product IDs previously.