Added descriptions to list items and updated docs.

Hey, 

I had a usecase where I wanted to give some general info about the model in its item view, and I found the field notes a bit clunky for that. So I added the option to give a model a description and it will show up in the Admin list item view of the model.
I also updated your docs with the description of this (see database page).

Some screens of what I'm talking about:
without description (same as now):
![without_desc](https://cloud.githubusercontent.com/assets/1830626/2789142/82d35690-cba8-11e3-8666-29e994712c7f.jpg)

with description:
![with_desc](https://cloud.githubusercontent.com/assets/1830626/2789141/77f06506-cba8-11e3-9f8d-fb8169d874d4.jpg)