listbox: Update docs
listbox: Update docs

Multiple selection is supported now.

davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,ahodesuka/gtk,grubersjoe/adwaita,jessevdk/gtk,Lyude/gtk-,chergert/gtk,Lyude/gtk-,jadahl/gtk,chergert/gtk,grubersjoe/adwaita,jessevdk/gtk,Adamovskiy/gtk,ahodesuka/gtk,jadahl/gtk,chergert/gtk,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,Adamovskiy/gtk,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,ahodesuka/gtk,chergert/gtk,alexlarsson/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,jessevdk/gtk,jessevdk/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jessevdk/gtk,jigpu/gtk,Lyude/gtk-,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,jadahl/gtk,Adamovskiy/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,jigpu/gtk,Lyude/gtk-,Lyude/gtk-,grubersjoe/adwaita,ahodesuka/gtk,Adamovskiy/gtk,jessevdk/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,chergert/gtk,jigpu/gtk,Lyude/gtk-,ahodesuka/gtk,jadahl/gtk,jigpu/gtk,ahodesuka/gtk,davidgumberg/gtk,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,grubersjoe/adwaita,alexlarsson/gtk,jessevdk/gtk