Document the difference between Store push() vs. createRecord() ect.

@jefflage I've attempted to address #1780 with this pull request. Could you please review it and let me know if you have any feedback?

![screen shot 2014-03-05 at 4 47 15 pm](https://f.cloud.github.com/assets/54056/2338960/cc5cd34c-a4af-11e3-8338-b0b6c319cdcd.png)