r.sim.water: add info to manual, add info about progress

[metzm]

• r.sim.water is a module with a number of important but not well documented parameters. This PR adds more information about some parameters to the manual.
• It takes quite some time to run a simulation, another message is added to the progress of the simulation.

This PR tries to add info requested in #4475 to the manual

[hmitaso]

This sentence may be confusing "The model is for shallow water flow, where "shallow" is defined by hmax"- Perhaps the paragraph could start with "the model tries to keep water "shallow" with maximum shallow water depth defined by hmax value (default is 0.3m). If hmax is reached then .... here goes the text that explains the increase in diffusion rate and related parameters .... The rest of the text that explains when these parameters may need to be adjusted sounds good to me.

[metzm]

This new description comes from #4475, maybe @petrasovaa can clarify? I am not sure if the model tries to keep water "shallow". After hmax is reached, water flow can more easily overcome obstacles like sinks. "shallow" might here refer not to water depth, but to noise or sinks in the DEM used as input.

[metzm]

@hmitaso New try to explain diffusion in order to keep water shallow and to overcome obstacles in ad725c7.