Admin reports docs

@nickpell 

Mostly docs from the PR to make things a little more clear
so ```intervals``` is a bit different than the original logic here - ```intervals``` creates a list of (start, end) dates that collectively exhaust the entire range of dates.  The original logic of this function, on the other hand, is to take a series of dates, and create a series of (date - delta, date) intervals, which may or may not overlap or have gaps (take months with 31 or 28 days).