adding fact mentioned in pull request mongodb#1551.

Author: Sam Kleinman

source/core/replica-set-oplog.txt

@@ -10,8 +10,8 @@ the data stored in your databases. MongoDB applies database operations
 on the :term:`primary` and then records the operations on the
 primary's oplog. The :term:`secondary` members then copy and apply
 these operations in an asynchronous process. All replica set members
-contain a copy of the oplog, allowing them to maintain the current
-state of the database.
+contain a copy of the oplog, in the :data:`local.oplog.rs` collection,
+which allows them to maintain the current state of the database.
 
 To facilitate replication, all replica set members send heartbeats
 (pings) to all other members. Any member can import oplog entries from