DOCS-378 added note on idempotency of oplogs

Author: Bob Grabar

source/core/replication-internals.txt

@@ -30,6 +30,16 @@ exceptional situations may cause secondaries to lag behind further. See
 All members send heartbeats (pings) to all other members in the set and can
 import operations to the local oplog from any other member in the set.
 
+Replica set oplog operations are :term:`idempotent`. Idempotency is
+required because of the following operations:
+
+- initial sync
+- post-rollback catch-up
+- sharding chunk migrations
+
+
+  .. TODO. Verify that "sharding chunk migrations" (above) requires idempotency
+
 For more information about the oplog, see :ref:`oplog sizing <replica-set-oplog-sizing>`.
 
 .. In 2.0, replicas would import entries from the member lowest