From the documentation you linked to:
detail
should be a string that identifies the widget or code doing the rendering, so that theme engines can special-case rendering for that widget or code.
The way I interpret that is that you might set it to "Wutaz-window-icon"
and then if theme writers needed to write a special case for your application, they could match that string.
However, the point is moot; as @MrEricSir points out, the function is deprecated.