Explicitly state structural restrictions of type Edm.Untyped

The name Edm.Untyped is misleading as this type has an underlying implicit type system that is just more flexible than CSDL's current system of primitive, collection, and structured types.

This underlying type system is a little underspecified, e.g. there's a somewhat hidden restriction - only stated in the JSON Format specification - that object member names have to be OData SimpleIdentifiers and are not just strings as in JSON.