API documentation

[mikeroswell]

I think this may be redundant with an existing issue or to-do in tasks.

Update API documentation in "main.py".

• Filter function should be clearly documented at the top (and vs. or, etc).
• current filter documentation for period_start_date and period_end_date a bit confusing although not terribly, each one can only take a date range but each works independently on the period start or end as expected. Documentation should just be clear about what the date range pertains to (e.g. any start date falling within /).