docs: Noted that test purchases have empty order ID.

Per bug, noted that orderID is blank for test purchases made with
the sandbox. Added this note both in the "testing" page and in the
reference for INAPP_PURCHASE_DATA.orderId.

See first comment for doc stage location.

bug: 19609418
Change-Id: I74ab54fdf69eeb32d331ea5900dc7582d8150dfa

docs/html/google/play/billing/billing_reference.jd

@@ -228,7 +228,9 @@ a purchase order.</p>
   <tr>
     <td>{@code orderId}</td>
     <td>A unique order identifier for the transaction. This identifier
-      corresponds to the Google payments order ID.</td>
+      corresponds to the Google payments order ID. If the order is a test
+      purchase made through the In-app Billing Sandbox, {@code orderId} is
+      blank.</td>
   </tr>
   <tr>
     <td>{@code packageName}</td>

docs/html/google/play/billing/billing_testing.jd

@@ -82,9 +82,11 @@ accounts.</p>
 <p>Once authorized for testing access, those users can make purchases without
 being charged.
 Test purchases are real orders and Google Play processes them in the same way as
-other orders. When purchases are complete, Google Play prevents the orders from
+other orders. However, the <code>orderId</code> field for test purchases is
+blank.
+When purchases are complete, Google Play prevents the orders from
 going to financial processing, ensuring that there are no actual charges to user
-accounts, and automatically canceling the completed orders after 14 days. </p>
+accounts, and automatically canceling the completed orders after 14 days.</p>
 
 <p class="note">
   <strong>Note:</strong> Test subscription purchases recur daily, regardless of