Add warning about Function.setter instances not saved

issue5651
review28321002

doc/ref/models/fields.rst

@@ -727,10 +727,13 @@ A function field can emulate any other given `field`.
     the value.
     The signature of the method id::
 
-        setter(ids, name, value)
+        setter(instances, name, value)
 
     where `name` is the name of the field and `value` the value to set.
 
+.. warning::
+    The modifications made to instances will not be saved automatically.
+
 .. attribute:: Function.searcher
 
     The name of the classmethod of the :class:`~trytond.model.Model` to search