I'd actually like to say that Python has great "documentation traditions" not just thanks to adoption of reStructuredText format, but a bit more. The greatest thing is an adoption of Sphinx [1] documentation engine. In Sphinx you write in extended (by special extensions) reStructuredText format, which is able to easily link to function definitions (with :func:`foo.bar.baz` or :class:`foo.bar.Baz`), other documents (with :doc:`other/doc`) and (most importantly) has extension called "autodoc" which goes and generates documentation for classes and functions automatically (by gathering it from docstrings and other things, similar to haddoc, if I'm not mistaken).

My main point is that thanks to adoption of "mixed documentation" techinique people usually tend to keep documentation with has these properties:

- it's up to date

- there's (most of the time) no need to keep separate API and non-API documentation

- API documentation can be easily be extended with long introduction without having to write it in source code (you can write it in document and then include autodoc)

I think Haskell's documentation would greatly benefit from (possible?) adoption of something like Sphinx, but I don't think it supports Haskell currently.