Improvements and addition of many links to the lib.dispersion.ns_cpmg_2site_star API documentation.

This is for the API documentation at:
http://www.nmr-relax.com/api/3.1/lib.dispersion.ns_cpmg_2site_star-module.html.
Added the 'NS CPMG 2-site 3D full' model to the lib.dispersion.ns_cpmg_2site_3d module docstring.