Add note about idempotency of replica set oplog operations

Addition in - http://www.mongodb.org/display/DOCS/Replica+Set+Internals

Replica set oplog operations are idempotent and need to be for a few cases:

• initial sync
• post-rollback catch-up
• sharding chunk migraitons?

Add to section - http://docs.mongodb.org/mangual/core/replication-internals/#oplog

Verify last point, link to "idempotent" glossary term