[docu][freeflow] Improve rans oneeq docu.
intercal and nicefrac don't work in the doxygen docu for me (https://dumux.org/docs/doxygen/master/a13883.html). kappa and sigma were not defined here, I think.
intercal and nicefrac don't work in the doxygen docu for me (https://dumux.org/docs/doxygen/master/a13883.html). kappa and sigma were not defined here, I think.