org.texi: complete org-crypt config example

Complete org-crypt config example with a hackish workaround
to disabling auto-save locally in a buffer:

# -*- buffer-auto-save-file-name: nil; -*-

Thanks to Bill Day for suggesting this.

doc/org.texi

@@ -13924,6 +13924,10 @@ To use org-crypt it is suggested that you have the following in your
   ;; to turn it off if you plan to use org-crypt.el quite often.
   ;; Otherwise, you'll get an (annoying) message each time you 
   ;; start Org.
+
+  ;; To turn it off only locally, you can insert this:
+  ;;
+  ;; # -*- buffer-auto-save-file-name: nil; -*-
 @end example
 
 Excluding the crypt tag from inheritance prevents already encrypted text