[NbtkExpander] fix gtk-doc documentation
[NbtkExpander] fix gtk-doc documentation

jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx