Mention that `delete-region' doesn't alter the kill ring.

src/editfns.c

@@ -3161,7 +3161,8 @@ It returns the number of characters changed.  */)
 
 DEFUN ("delete-region", Fdelete_region, Sdelete_region, 2, 2, "r",
        doc: /* Delete the text between START and END.
-If called interactively, delete the region between point and mark.  */)
+If called interactively, delete the region between point and mark.
+This command deletes buffer text without modifying the kill ring.  */)
   (Lisp_Object start, Lisp_Object end)
 {
   validate_region (&start, &end);