I was browsing around the CouchDb wiki yesterday when I saw Damien Katz' hilarious description of how technical documentation really gets written. You know, in the real world:

Welcome to the world of technical documentation!

The situation you are in is no different from any other tech writer. The technical writing process:

1. Ask engineer how the damn thing works.
2. Deafing silence.
3. Crickets.
4. Tumbleweed.
5. Just start writing something. Anything.
6. Give this something to the engineer.
7. Watch engineer become quite upset at how badly you've missed the point of everything.
8. As the engineer berates you, in between insults he will also throw off nuggets of technical information.
9. Collect these nuggets, as they are the only reliable technical information you will receive.
10. Try like hell to weave together this information into something enlightening and technically accurate.
11. Go to step 6.

Ok, you're not the doc writing type. That's okay, neither am I. However, people are already working to make this better, and I will continue to do so.

I think I've been on both ends of this exchange at some point in my career. It's funny because it's true. I'm sure I've read descriptions exactly like this on Mike Pope's blog.