widget: Clarify docs on gtk_widget_set_double_buffered()
widget: Clarify docs on gtk_widget_set_double_buffered()

Also make them more scary so people really really don't use it as a
random knob when trying to make things go fast.

//bugzilla.gnome.org/show_bug.cgi?id=750505

chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,grubersjoe/adwaita,chergert/gtk,Adamovskiy/gtk,davidgumberg/gtk,chergert/gtk,Adamovskiy/gtk,chergert/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk