Updated copy on section error to include student enrollment example.

@rocha @clintonb @lamagnifica 

![screen shot 2014-10-01 at 11 23 23 am](https://cloud.githubusercontent.com/assets/5265058/4477557/f7be1b30-497e-11e4-97c1-0bf878436820.png)

Sure, I can do that.  @clintonb and I considered that and ended up going the other way, but it wasn't definitive.