docs: Added note - test subscriptions cannot use reserved IDs

The "Testing In-app Billing" page now includes a note which explains
that test subscription purchases must use real purchase IDs, not
any of the IDs reserved for testing one-time in-app purchases.

Bug: 27352434
Change-Id: I625178c3517ce9940bc4746d99f1d506fd5e944b
1 file changed