DOC: Unclear documentation for Period Construction
The documentation is unclear as to what the date passed to a `Period` means: whether it is the start date or the end date of the period. Convention is that fiscal quarters are usually stated by the date the quarter ends, whereas weeks are many times stated by the day week starts. This is especially unclear given that the default `freq='W'` is `W-SUN` which means the "week ending on Sunday" whereas one convention is to show weeks starting on Sunday. 

I would recommend clearing this up in the documentation. 
