docs: transaction doesn't have property succeeded

The docs still mention the succeeded property on transactions run with with even though it has been removed in ed0762b. It's not clear to me how to check for a successful transaction now. I suspect it throws an exception otherwise, which mirrors ndb behaviour, but I still check for _status == 3 just in case.

@pdknsk

@pdknsk Thanks for pointing it out. I was hoping it was in an old docs build, but I confirmed it is present in HEAD.

As for how you can check it going forward, see my comment in the PR.

The context manager doesn't swallow exceptions. So __exit__ing the context manager without a failure is equivalent to success.

The only reason we kept the _status around is so that a Transaction object is "tombstoned" after one use. In other words, so people can't mistakenly try to re-enter a transaction.