Updated documentation for the ofRectangle class.

As part of my trying to learn OpenFrameworks, I've been reading through the code and I trying to understand some of the simpler classes.  I'd love for someone to read through this and make sure that I haven't misstated anything.  I'm happy to fix up things as much as I can.

Also, I did not update the `::operator =`, or the `>>` and `<<` functions—I'm not sure how to phrase properly what they do.  I'd love suggestions of proper phrasing.