It can sometimes be useful to look through past release notes to see when new features became available or to check for known issues at a particular point in time.
The official Release Notes are available by downloading the stack from within Simplicity Studio or the Software Downloads area of the Portal and checking in the Documentation folder. This does require a large download per stack.
For your viewing pleasure, we have provided a dropbox folder which contains these files organized by stack here.
If you are looking for our Zigbee Compliant Platform documentation, that can be found here as they become available.