Add documentation about ip_address.iad_type column

[TheRedTrainer]

As commented in rdap-sql-provider issue 13 , the iad_type value is not loaded from database when the whole ip_address object is queried because it is only used when a partial search is executed, it is not used in any setter or getter.

It is recommended to explain this situation on the database script, web-site and code documentation, and on the column description.