with Pango markup.
Also see gtk.tooltip.Tooltip.setMarkup.
This is a convenience property which will take care of getting the tooltip shown if the given string is not null: gtk.widget.Widget.hasTooltip will automatically be set to true and there will be taken care of gtk.widget.Widget.queryTooltip in the default signal handler.
Note that if both gtk.widget.Widget.tooltipText and gtk.widget.Widget.tooltipMarkup are set, the last one wins.