I think this particular piece of advice from Joel is misguided. Unfortunately, very few of us are as funny as we think we are. (In fact, I think Joel is a pretty funny and engaging writer in general, but for some strange reason his example "funny" spec is more annoying than funny.)
For an example of engaging documentation, I like Dive Into Python. It's well-written and well-structured, and the author knows who his audience is.
The best things you can do to make your documentation engaging are:
[All documentation] Figure out who you're writing for. The answer is not "everyone" or even "all technical people" in your organization.
[All documentation] Get yourself a copy of Style: Toward Clarity and Grace and load the entire book into your brain. :)
[API documentation] Be sure to keep in all the things that API reference documentation typically fails to include. What is this thing? Why should I use it? Are there any caveats I should know about? What are the typical/expected ways to use it? What are the bad/pathological ways of using it? Finally, provide one or more full-fledged examples that exercise your data access layer to do something non-trivial.