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>
En général on évite les mots inutiles dans la doc genre gets, shows, indicates...

`does this definition introduce the `MClass`?`