gtk-message-dialog-format-secondary-markup (dialog message &rest args)

dialog -- a gtk-message-dialog widget
message -- a markup string, see Pango markup format, or nil
args -- the arguments for message

Sets the secondary text of the message dialog to be message with the arguments in args, which is marked up with the Pango text markup language. Note that setting a secondary text makes the primary text become bold, unless you have provided explicit markup. Due to an oversight in the C implementation, this function does not escape special XML characters like the gtk-message-dialog-new-with-markup function does.