Added the 'NS R1rho 3-site' models to the relax user manual.

This is for the 'NS R1rho 3-site' and 'NS R1rho 3-site linear' dispersion models.

This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_relax_manual.