The docstring doesn't give details, but many docstrings don't. The docs themselves do; not sure it's a good idea to expand every docstring into complete documentation; if you don't know the function you read the real docs, the docstrings are useful as a reminder for stuff like argument order and names, higher logic, etc., without getting into details.

cmp is almost certainly not going to be documented further in the docstrings; 2.7 is in bug fix only mode IIRC (and even then, mostly security fix only), and tweaking docstrings that aren't wrong, just less detailed than you might like isn't likely to meet the bar.