add more prose for AKI
diff --git a/docs/x509.rst b/docs/x509.rst
index 932801b..ade8076 100644
--- a/docs/x509.rst
+++ b/docs/x509.rst
@@ -663,14 +663,21 @@
:type: bytes
+ A value derived from the public key used to verify the certificate's
+ signature.
+
.. attribute:: authority_cert_issuer
:type: :class:`Name` or None
+ The :class:`Name` of the issuer's issuer.
+
.. attribute:: authority_cert_serial_number
:type: int or None
+ The serial number of the issuer's issuer.
+
.. class:: SubjectKeyIdentifier
.. versionadded:: 0.9