[gtk] docs: Remove a mention of Gtkinvisible

[Matthias Clasen <matthiasc src gnome org>]

commit eb1310effe1470a0659f77b34124835ff707187d
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Feb 24 10:40:17 2019 -0500

    docs: Remove a mention of Gtkinvisible

 gtk/gtkwidget.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkwidget.c b/gtk/gtkwidget.c
index 80022c2f87..b782022d6f 100644
--- a/gtk/gtkwidget.c
+++ b/gtk/gtkwidget.c
@@ -6350,8 +6350,9 @@ gtk_widget_get_has_surface (GtkWidget *widget)
  *
  * Determines whether @widget is a toplevel widget.
  *
- * Currently only #GtkWindow and #GtkInvisible are toplevel widgets.
- * Toplevel widgets have no parent widget.
+ * Currently only #GtkWindows are toplevel widgets.
+ * Toplevel widgets have no parent widget and implement
+ * the #GtkRoot interface.
  *
  * Returns: %TRUE if @widget is a toplevel, %FALSE otherwise
  **/