added some comments

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@4493 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/PropertySheet/Reference.py

@@ -52,6 +52,9 @@ class Reference:
             'description' : 'A reference which allows to unify multiple objects',
             'type'        : 'string',
             'mode'        : 'w' },
+        # Do not use the property below - it may be replaced by calculated
+        # references made out of reference, version_reference, portal_type, etc.
+        # ex. PROD-33456724-3
         {   'id'          : 'document_reference', # XXX ERROR - we already have a reference
             'description' : 'The references of the document itself',
             'type'        : 'string',