cherry-pick from mnc-io-docs: docs: Updated descriptions for CSV file

settings within IAB.

The section titled "Adding a batch of items to a product list," found
within the "Administering In-app Billing" page on the DAC site, now
contains up-to-date descriptions of the different settings within the
CSV files used with product lists within the Google Play Developer
Console.

Bug: 28767053
Change-Id: I578aaacdaa866d9fbac8a087f22eb1c3617437ba