Document 'expected' field values.

dbase_setup.rst

@@ -801,7 +801,12 @@ Element content: Field name + optional list <optlist> of allowed values, e.g.::
 
 * required	
 
-  * Sets if data is required for this field, 'yes' or 'no', default 'yes'.	
+  * Sets if data is required for this field, 'yes', 'no', or 'expected', 
+    default 'yes'. If set to 'expected', the value cannot be left empty when
+    batch adding an isolate record or using the submission system, but a null 
+    value can be explicitly set using the value 'null'. The use of this is to
+    encourage submitters to include a value for this field if it is available,
+    while still allowing empty values if it is not.	
 
 * userfield