The "admin" and "config" databases each contain a handful of system collections, holding replica set or sharding state. I think the admin and config DBs were historically distinguished by different locking rules, but these days there is no rationale for keeping them separate.

Let's consider merging them. However, it seems likely that merging them is more difficult than it's worth. In that case, let's add to the Replication Architecture Guide some information about these two databases, and a policy for where new system collections are placed as we develop new features in the future.