Added doc about Function field

doc/models.rst

@@ -409,6 +409,25 @@ used in the method definition, hence adapting the way the method is
 called.
 
 
+Another way to tackle Function implementation is to pass a dictionary
+to the ``args`` argument on the field definition. It will be forwarded
+to the function call:
+
+.. highlight:: python
+
+::
+
+  state = fields.Function(
+      'get_state', type='selection', string='Total Service',
+      args={'key':'value'},
+      selection=[('draft','Draft'),('done','Done')],
+      )
+
+  def get_state(self, cursor, user, ids, names, arg, context=None):
+      # [...]
+      if arg.get('key'):
+          pass # do something with 'value'
+
 
 Search on function fields
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