Fixed #17272 -- Documented that non-editable fields are skipped during model validation.

Thanks mitar for the report.

docs/ref/models/fields.txt

@@ -226,7 +226,8 @@ See that documentation for other caveats.
 .. attribute:: Field.editable
 
 If ``False``, the field will not be displayed in the admin or any other
-:class:`~django.forms.ModelForm`. Default is ``True``.
+:class:`~django.forms.ModelForm`. They are also skipped during :ref:`model
+validation <validating-objects>`. Default is ``True``.
 
 ``error_messages``
 ------------------