http://docs.mongodb.org/master/reference/method/db.upgradeCheck/
http://docs.mongodb.org/master/reference/method/db.upgradeCheckAllDbs/

1. Clarify the performance impact to run db.upgradeCheck() and db.upgradeCheckAllDbs()
   db.upgradeCheck() and db.upgradeCheckAllDbs() validates all the document in the given collections. This looks like a collection scan, and might affect the performance on a production environment. If it is true, can you add this warning in the document?
2. If it is a sharded cluster/replica set, it would be nice to document which nodes we should run db.upgradeCheck() or db.upgradeCheckAllDbs().