The architecture guide describes replication progress before we pushed all secondary state information into the TopologyCoordinator. See
SERVER-26990, where we got rid of SlaveInfo in ReplicationCoordinatorImpl and consolidated everything into memberHeartbeatData.
SERVER-29077, we also renamed memberHeartbeatData to memberData since it contains more than just heartbeat data.
We should make sure the architecture guide reflects these changes since they were certainly true as of 4.2 (changed after 3.6).