diff --git a/runtime/doc/editing.txt b/runtime/doc/editing.txt
index 11f5748..32848ac 100644
--- a/runtime/doc/editing.txt
+++ b/runtime/doc/editing.txt
@@ -1366,6 +1366,9 @@
 automatically to the method used when that file was written.  You can change
 'cryptmethod' before writing that file to change the method.
 
+When writing an undo file, the same key and method will be used for the text
+in the undo file. |persistent-undo|.
+
 						*E817* *E818* *E819* *E820*
 When encryption does not work properly, you would be able to write your text
 to a file and never be able to read it back.  Therefore a test is performed to