docs: Added descriptions of IAB subscription-specific settings.

The "Adding items one at a time to a product list" section on the
"In-app Billing Administration" page now includes a new sub-section,
"Adding subscription details," which describes the settings that developers
can configure when creating a new subscription associated with an app.

Bug: 28765727
Change-Id: Ie24c5dc505b11ebf9db0798add1a3150eaee59bc