Better document the Stats model.

[khandelwal]

Current Stats model: https://github.com/18F/foia-hub/blob/master/foia_hub/models.py#L144

I think there a couple things here that might be worth changing:

• [ ] Rename Stats to Statistics, there's no benefit in shortening it I think
• [x] The docstring should be more descriptive. What statistics are stored here, which reports to they come from?
• [ ] The field 'median' should probably be renamed to reflect what statistic it is a median of.
• [x] There should be some documentation around STAT_TYPE. What is the definition of simple? What is the definition of complex?
• [x] What units are the median in?