Document stability for olx properties

The stability level will be used to determine what can change between minor releases.

This adds a ton more "experimental" labels to the doc.  The next step is to mark more olx properties as stable  (see #2578).



![docs](https://cloud.githubusercontent.com/assets/41094/4006747/87acc452-29b4-11e4-9c04-54fe6c9fca38.png)