docs: Improve deprecation notices for GtkImageMenuItem properties
docs: Improve deprecation notices for GtkImageMenuItem properties

Should help porting code away from the deprecated GtkImageMenuItem
class.

https://bugzilla.gnome.org/show_bug.cgi?id=750718

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