Show some love for fetchNavigation()

So tonight, for the second time, I did have to read the code from `__buildNavigation()` in `AdministrationPage` class in order to find out that the thing I wanna do was already there... But not in the doc!

Since the doc is separated from the actual code, this one is though for maintenance.

So I pimp the doc a bit. Hope you like it. It think it now covers everything! Feel free to remix. At least, it's an improvement ;)