gdk: Document the behavior of device grabs on slave devices
gdk: Document the behavior of device grabs on slave devices

This may be useful for certain applications, so document it explicitly.

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