VODataService: documenting foreign keys

[Ray Plante]

Hi RWGers,

We've had some great Registry sessions so far here in Strasbourg, 
including the one in which we reviewed VODataService.  Since that session, 
Gerard and Pat came up with what I think is a very good suggestion:  add 
optional utype and description elements as children of the foreignKey 
element.  The purpose is to allow table providers to document the purpose 
of the foreign key, further helping a user understand how to join tables.

I have not added this to the schema and spec document, but I plan to by 
the end of the week.  If you have any comments--particularly objections or 
concerns--please share them this week.

cheers,
Ray