I agree with “Anything that you can’t tell from the method’s signature”. It might also mean to explain what a method/function returns.
You might also want to use Sphinx (and reStructuredText syntax) for documentation purposes inside your docstrings. That way you can include this in your documentation easily. For an example check out e.g. repoze.bfg which uses this extensively (example file, documentation example).
Another thing one can put in docstrings is also doctests. This might make sense esp. for module or class docstrings as you can also show that way how to use it and have this testable at the same time.