That is something that MUST exist in documentation. If you're putting it within the configuration file itself it can go stale or simply deleted one day and you would have no idea what occurred after the fact without looking at a revision history.
I'm a bit puzzled by documentation seems like a bad word around here; it has its place and I'm not suggesting writing up a huge document. It takes only a couple of minutes to drop some text in Word / Google Docs / Wiki / your flavor and put it someplace accessible for your entire team.
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.
I'm not refuting the value of documentation, I'm just acknowledging the referential integrity issues with it. All too frequently I've encountered cases where a config file gets updated but the wiki doesn't. Since all configuration files are version-controlled, I never run into a situation where a file changes and I don't know why (well, discounting bad commit messages). But, a documented wiki wouldn't help in that case anyway. A comment is documentation too ...
I'm a bit puzzled by documentation seems like a bad word around here; it has its place and I'm not suggesting writing up a huge document. It takes only a couple of minutes to drop some text in Word / Google Docs / Wiki / your flavor and put it someplace accessible for your entire team.