transaction 16MB limits add details

Author: kay-kim

source/core/transactions-production-consideration.txt

@@ -29,6 +29,9 @@ cleanup process.
 Oplog Size Limit
 ----------------
 
+When the transaction commits, a single oplog entry is created if the
+transaction contains any write operations. That is, the individual
+operations in the transactions do not have a corresponding oplog entry.
 The oplog entry for the transaction must be within the BSON document
 size limit of 16MB.