Update error message to link to repo

Co-authored-by: Bart de Water <[email protected]>

lib/job-iteration/iteration.rb

@@ -238,7 +238,7 @@ def assert_valid_cursor!(cursor)
       Deprecation.warn(<<~DEPRECATION_MESSAGE, caller_locations(3))
         The Enumerator returned by #{self.class.name}#build_enumerator yielded a cursor which is unsafe to serialize.
 
-        TBD MENTION SERIALIZERS!
+        See https://github.com/Shopify/job-iteration/blob/main/guides/custom-enumerator.md#cursor-types
 
         This will raise starting in version #{Deprecation.deprecation_horizon} of #{Deprecation.gem_name}!"
       DEPRECATION_MESSAGE