print_options: warn that the files are generated.

doc/print_options.c

@@ -112,6 +112,8 @@ int main(int argc, char **argv)
     if (argc < 2)
         print_usage();
 
+    printf("@c DO NOT EDIT THIS FILE!\n"
+           "@c It was generated by print_options.\n\n");
     if (!strcmp(argv[1], "format"))
         show_format_opts();
     else if (!strcmp(argv[1], "codec"))