[AlisdairO]

Documentation isn't a bad word - I regularly make documents to describe the architecture of my projects, along with their an outline of how to get into the codebase, for example. Comments are also documentation, though, and in this case comments are the right place. If I'm looking at a configuration option and thinking 'That seems a bit odd. Why do we have that set?' I want to be able to see the reasoning right there, not have to hunt through a wiki in the hope that someone has put their reasoning somewhere.

In my experience, comments are less likely to go stale than wiki docs.