I'm working on correcting the documentation. The `ridder` and `bisect` methods have a very different convergence criterion:

    tol = xtol + rtol*(fabs(xa) + fabs(xb));

And this doesn't even change from one iteration to the next. As far as I can tell these two solvers are not recursive. Should this be changed to match the convergence criterion for `brentq` and `brenth`?