Clarify the field usage guidelines

The note explaining not to use the Nova versions of object field types
mentions that they will be removed, implying that the types listed
there, such as Dict and List should not be used. This update clarifies
that it is only the Nova versions of these fields that will be removed,
and that it is perfectly fine to use the o.vo field types.

Change-Id: I4ae2392e97f5cbe59e5c9c94c06c48a5d9c9d7d3

nova/objects/fields.py

@@ -72,7 +72,8 @@ PCIAddressField = fields.PCIAddressField
 # NOTE(danms): These are things we need to import for some of our
 # own implementations below, our tests, or other transitional
 # bits of code. These should be removable after we finish our
-# conversion
+# conversion. So do not use these nova fields directly in any new code;
+# instead, use the oslo.versionedobjects fields.
 Enum = fields.Enum
 Field = fields.Field
 FieldType = fields.FieldType