documentRef
documentRef — Reference to an external document
Content model
documentRef :  [docID],  refID
     EMPTY
Attributes
- docID
-  (optional) - the internal XML identifier of a reference definition element 
- refID
-  - the internal XML identifier of a reference definition element 
Future plans for this element
      The 'docID' attribute is deprecated; it has been renamed 'refID'
      to match it's use in the 'reference' element. This attribute
      will be removed in a future version of DAVE-ML.