Invalid returnOrderId in bitshares/market sell/buy type annotation

[Algruun]

In annotation returnOrderId is string

:param string returnOrderId: If set to "head" or "irreversible" the call will wait for the tx to appear in
                                        the head/irreversible block and add the key "orderid" to the tx output

But it used only like a bool

        if returnOrderId:
            # Make blocking broadcasts
            prevblocking = self.blockchain.blocking
            self.blockchain.blocking = returnOrderId

        tx = self.blockchain.finalizeOp(order, account["name"], "active", **kwargs)

        if returnOrderId:
            tx["orderid"] = tx["operation_results"][0][1]
            self.blockchain.blocking = prevblocking

[xeroc]

This is a rather hacky details.
Basically, the blocking variable is read in transactionsbuilder.TransactionBuilder.broadcast() and results in instanciating blockchain.Blockchain with the value of blocking as mode. That results in the blockchain.BLockchain.blocks() call to only look for blocks that are either "head" (most recent) or "irreversible".

I am open for suggestions to make those more clear and easier to maintain