Add documentation for why retrieving Subscription Information OfferIds and BasePlan for Purchases is not possible.

[EvanPartidas]

I think the FAQ could seriously use the following section.

How can I get the Subcription OfferId/BasePlan from the Purchase History?

This is not possible because:

• The Google Play Billing Library as of v7 doesn't support any way to get subscription info from a purchase. Purchase object does not have details about Subscription
• The Google Play Developer API, which grants access to this information, cannot be called client-side without huge security risks. Subscription Best Practices.

The only solution is to create a backend that calls the Google Play Developer API on the client's behalf.