The default value for the fit_intercept parameter is True for all linear models, but this default value is documented for Lasso/LassoCV estomators, but not for Ridge/RidgeCV. Maybe replace the string fit_intercept : boolean with fit_intercept : boolean, default True in Ridge/RidgeCV documentation, too? Or maybe better with bool, default True as noted in issue #14404 ?

Ready to do it, already done in my fork.