add the oid required by the extensiontype interface
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 14fc37c..3051308 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -1871,6 +1871,12 @@
identifier. For more information about the use of this extension see
:rfc:`5280`.
+ .. attribute:: oid
+
+ :type: :class:`ObjectIdentifier`
+
+ Returns :attr:`~cryptography.x509.oid.ExtensionOID.POLICY_CONSTRAINTS`.
+
.. attribute:: require_explicit_policy
:type: int or None