window: Add comment documenting the target widget checks during window drag
window: Add comment documenting the target widget checks during window drag

Was suggested during review of
https://bugzilla.gnome.org/show_bug.cgi?id=745562#c2

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