When does System.ApplicationReadWriteMode apply and what is five minute upgrade?

It looks like the doc is out of date (I've prodded the doc writers!).

Current info from the PMs: What USED to happen was that there would be a few minutes downtime, then several hours in read-only mode. That changed with Winter '12; now, the instance will be unavailable for up to five minutes (most instances less than one minute) and then it's live with the new release. System.ApplicationReadWriteMode is no longer used to support releases.