neo: Complete the documentation of neo.nit.

The documentation of the database model missed many attributes and
relationships. I only added the missing documentation.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
Je ne comprend pas plus que toi. J’ai simplement copié-collé la documentation de [`MClass.mclass_type`](http://nitlanguage.org/doc/nitc/property_workspace__MClass__mclass_type.html#workspace__workspace__model__model__MClass__mclass_type).