Update a document in DynamoDB, with a hash primary key to identify the
document, and using the specified config.

C# |
public Document UpdateItem( Document doc, Primitive hashKey, UpdateItemOperationConfig config )

- doc (Document)
- Attributes to update.
- hashKey (Primitive)
- Hash key element of the document.
- config (UpdateItemOperationConfig)
- Configuration to use.

Null or updated attributes, depending on config.
