Note this is not to change the validity of such configs, so the warning should not occur during validation, only when the config is actually applied by the node as being the current active replset config (regardless of whether it has come from local.system.replset during startup initialisation, or the replSetReconfig command, or a heartbeat). It should also not occur when getLastErrorDefaults are consulted to be applied to an operation (this would cause significant log spam).

Could be warning() or startupWarning(). startupWarning() has the advantage of better visibility (eg. shows up in the shell and Cloud/Ops Manager), but at the cost of not being able to be cleared except via a mongod restart (SERVER-19790).