switch reasons to frozenset
diff --git a/docs/x509.rst b/docs/x509.rst
index f9992e2..86673e3 100644
--- a/docs/x509.rst
+++ b/docs/x509.rst
@@ -835,7 +835,7 @@
 
     .. attribute:: reasons
 
-        :type: list of :class:`ReasonFlags` or None
+        :type: frozenset of :class:`ReasonFlags` or None
 
         The reasons a given distribution point may be used for when performing
         revocation checks.