Add extra comment why deferred fields are skipped with ___ lookups

Author: vdboor

polymorphic/query.py

@@ -393,6 +393,9 @@ class self.model, but as a class derived from self.model. We want to re-fetch
                         # now a superclass of real_concrete_class. Thus it's
                         # sufficient to just use the field name.
                         translated_field_name = field.rpartition('___')[-1]
+
+                        # Check if the field does exist.
+                        # Ignore deferred fields that don't exist in this subclass type.
                         try:
                             real_concrete_class._meta.get_field(translated_field_name)
                         except FieldDoesNotExist: