Website documentation... or lack of it!

A new student has started working in my office, due to the fact that he is taking over my position fully in 4 weeks (when I'll be dragged kicking and screaming from Switzerland). I was horrified to find that very little technical information there is about the web application I have devoted so much time to over the past year.

I now find myself having to teach the student with demonstrations and looking through the code together. I am trying to piece together a technical manual at the same time, to act as a reference for him when I leave.

It makes me wonder.. How many web sites or web applications actaully have a technical guide?

I'm not after a fancy class diagram either!