Updated the 'MMQ 2-site' model description in the manual.

The R2_DQ = R2_ZQ = R20 assumption is now explained.
Added the 'NS MMQ 3-site' models to the relax user manual.

This is for the 'NS MMQ 3-site' and 'NS MMQ 3-site linear' CPMG 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.