GtkOverlay: Update the description
GtkOverlay: Update the description

State that the overlays are placed wrt to the GtkOverlay, not
with respect to the main widget. This makes a difference for
small main widgets which are not configured to fill the entire
GtkOverlay.

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