add test to confirm IDNA2003 is disallowed

document that this is a possible exception
diff --git a/docs/x509.rst b/docs/x509.rst
index c8505a8..c570f19 100644
--- a/docs/x509.rst
+++ b/docs/x509.rst
@@ -313,6 +313,9 @@
         :raises cryptography.x509.UnsupportedGeneralNameType: If an extension
             contains a general name that is not supported.
 
+        :raises UnicodeError: If an extension contains IDNA encoding that is
+            invalid or not compliant with IDNA 2008.
+
         .. doctest::
 
             >>> for ext in cert.extensions: