Rich html content can be displayed in the cell tooltips by using attr property in column.render callback.
render: function( ui ){
return {
attr: {
title: "<b>Rich</b> <i>Text<i>!"
}
}
}
and creating pqTooltip in the create callback in main grid options.
create: function (evt, ui) {
this.widget().pqTooltip();
}