Surely documentation that is "not worth the bits it is printed on" doesn't count as good documentation!

Far too often what the original author documented was already obvious to me based on the code.

Sounds like the difference between comments and documentation. Documentation tells you *what* the software does - the API, perhaps a high-level description of the algorithm. Documentation exists to help people who want to use your code (including other programmers) as well as maintainers. Comments are about *how* it does the job, and help explain much lower level stuff, eg "treat \ character as special", "these constants copied from MySQL headers", and are only for maintainers. Even so, they shouldn't say things like "add one to $i".