Document `PhasorDynamics::Genrou` model

[pelesh]

There are slight differences between documentation for the PhasorDynamics::Genrou model and its current implementation. It would be helpful to update README file for the PhasorDynamics::Genrou model to match precisely the equations as coded.

Also, it would be helpful to review documentation for the generic synchronous machine model and remove any duplication with Genrou docs, just to avoid any confusion.