[IMP] base: improved doc for states attribute of fields

See #1527

openerp/fields.py

@@ -125,8 +125,12 @@ class Field(object):
         :param default: the default value for the field; this is either a static
             value, or a function taking a recordset and returning a value
 
-        :param states: a dictionary mapping state values to lists of attribute-value
-            pairs; possible attributes are: 'readonly', 'required', 'invisible'
+        :param states: a dictionary mapping state values to lists of UI attribute-value
+            pairs; possible attributes are: 'readonly', 'required', 'invisible'.
+            Note: Any state-based condition requires the ``state`` field value to be
+            available on the client-side UI. This is typically done by including it in
+            the relevant views, possibly made invisible if not relevant for the
+            end-user.
 
         :param groups: comma-separated list of group xml ids (string); this
             restricts the field access to the users of the given groups only