All cv should say that the default is stratified for classification, and then a list of available objects is in the cross_validation module.
Currently, some say

A cross-validation generator to use. If int, determines the number of folds in StratifiedKFold if y is binary or multiclass and estimator is a classifier, or the number of folds in KFold otherwise. If None, it is equivalent to cv=3.

I find the "y is binary or multiclass" confusing and I think it is not actually true, so I'd remove that part.

It's easy to find all docstring with a git grep "cv :"