Created attachment 5080 Use-xfce_dialog_show_help.patch There is no ThunarHelp anymore, so use xfce_dialog_show_help() instead.
I suspect this patch should use show_help_with_version since Thunar has its own versioning scheme, but it should be applied nevertheless.
Fixed with support for versioned docs: http://git.xfce.org/xfce/thunar-volman/commit/?id=8d3cc36034cefa5f4f5edd0b90d0a8cbedbadf8f