Remove an XXX an explain why in comments.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@1743 71dcc9de-d417-0410-9af5-da40c76e7ee4

neo/storage/transactions.py

@@ -201,7 +201,9 @@ class TransactionManager(object):
             Abort a transaction
         """
         if tid not in self._transaction_dict:
-            # XXX: this happen sometimes, explain or fix
+            # the tid may be unknown as the transaction is aborted on every node
+            # of the partition, even if no data was received (eg. conflict on
+            # another node)
             return
         transaction = self._transaction_dict[tid]
         has_load_lock = transaction.isLocked()