Fixed typos and suggested changes to documentation

And some random remarks:

- You could maybe reference tgext.datahelpers and the Django FileField/Storage which are a bit similar at least for local storage
- You could use the "Interactive Interpreter" doc style, where you display the output of the print statements and so on
- Some Sphinx `:class:`-Links are not rendered correctly as links
- Justified text doesn't look so good on the API doc page