update py3status doc for v2

As requested by email, here is a pull request to update py3status documentation for its v2.
That's a point I've also thought about and kept in mind while developing this version. I still remain confident and think it's ok for these main reasons :

- You've proven over the years that i3status is a solid and breaking free program imho with limited changes.
- Py3status actually only adds **one** parameter that could conflict with any new parameter you could add on future releases (on_click X = "i3 cmd").
- All the non-py3status parameters are passed as-is to i3status. Shall you add/remove new ones to existing modules this won't affect anything.
- Worst case scenario would be a new module added to i3status which would in turn be used by an old py3status v2 user. I think it's fair to ask for a simple update of py3status in this case. I reckon it also happens with i3 components themselves where the legitimate answer is to ask the user to just update his packages (we're talking about edge cases here).

As I hope you'll agree with me, the benefits of the user friendly features and the dont-add-yet-another-config-file philosophy far overcome this limited problem :)