add note about parallel operations (#94)

Author: rachel-mack

source/write/transactions.txt

@@ -34,6 +34,10 @@ transaction that meets an expectation of atomicity, consistency, isolation, and
 guarantees that the data involved in your transaction operations remains
 consistent, even if the operations encounter unexpected errors.
 
+.. note::
+
+   The {+driver-short+} does not support running parallel operations within a single transaction.
+
 When using the {+driver-short+}, you can create a new session from a ``mongocxx::client`` instance.
 Then, you can use the resulting ``mongocxx::client_session`` instance to perform transactions. 
 We recommend that you reuse your client for multiple sessions and transactions instead of