revise STO chapter for v4.2.1 release

Correct few text typos and add a reference to the logical keys to activate the stochastic components for lateral diffusion, horizontal pressure gradient, ice strength, trend, and tracers dynamics.

However, as explicitly stated in the manual only the stochastic parametrization of equation of state is fully available and tested and I think that the STO description should be left as is, until someone will reboot this activity.