title: Transactions and ConcurrentModificationException documentation ambiguity
body: <p>The latest documentation here <a href="http://code.google.com/appengine/docs/java/datastore/transactions.html" rel="nofollow">http://code.google.com/appengine/docs/java/datastore/transactions.html</a> states:
  "If your app receives an exception when submitting a transaction, it does not always mean that the transaction failed. You can receive DatastoreTimeoutException, ConcurrentModificationException, or DatastoreFailureException exceptions in cases where transactions have been committed and eventually will be applied successfully. Whenever possible, make your Datastore transactions idempotent so that if you repeat a transaction, the end result will be the same."</p>
  
  <ol>
  <li>How is it possible to know if a transaction succeeded or not?</li>
  <li>What am I going to do after catching ConcurrentModificationException? How do I know if I have to retry?</li>
  <li>How would you recommend making, for example, payment transactions idempotent without any idea whether it was successful? </li>
  </ol>