support unrecognized extensions in x509
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index df17f95..01a5d94 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -2435,7 +2435,8 @@
 
 .. class:: UnsupportedExtension
 
-    This is raised when a certificate contains an unsupported extension type.
+    This is raised when a certificate contains an unsupported extension type
+    that is marked critical.
 
     .. attribute:: oid