I definitely concur with the statement on organizing by release cycle (or perhaps using the PHP convention of showing deprecation and support version information for different calls).

I recently ran into issues with replicaSet myself; finding it hard to locate documentation on using user-based authentication. It boiled down to me eventually locating the necessary info on the Master-Slave page for ensuring I did a db.addUser() on the slave's local db. All in all, I'm much looking forward to a rewrite of the documentation.