When set to `traditional' (the default), provide a more basic preview
according to `register-preview-delay'; this preserves the traditional
-behavior of Emacs 29 and before."
+behavior of Emacs 29 and before.
+
+Setting this variable with `setq' has no effect; use either `setopt'
+or `customize-option' to change its value."
:type '(choice
(const :tag "Use preview" t)
(const :tag "Use preview and exit by pressing register name" insist)