I know it first hand from my own experience with NxBRE: writing and maintaining the 55+ pages PDF guide is not only a significant effort, but one of a very different nature than the effort of developing the product itself.
The wiki-based knowledge base is a good alternative: less formal, built from users questions and easy to maintain, it is definitively a viable approach for documenting small scale projects like mine.
This is also something learned on the field, as I have to work a lot with open source solutions. A pristine and up to date documentation is still an exception, at least for non-company backed projects. After hours of trial and error, fighting with an on-line documentation that was sometimes outdated (many examples would simply not work) and sometimes too advanced (showing features available in snapshot builds only), I came to consider recommending a paid-for solution, just for the sake of having someone to blame if the documentation was bad.
But that would have been too easy to surrender that way! Instead, I reverted to the more courageous tactics of the open source addict:
- explore the provided running examples,
- if not enough, browse the test cases,
- if still not enough, trace debug with the source code attached to the IDE.