[development] update hooks/APIs documentation

[Bèr Kessels]

Hello, 

Is there documentation somwhere about the update hooks and APIs. I tried to 
understand them from reading the code. And write some myself, but found this 
too hard, to do withou the solid understanding of the actual code. 

I guess that an example module upgrade script will suffice, for now. 

Is anything already online? Does such an example exist?

Bèr