[gtk] Document gdk_toplevel_set_modal

[Matthias Clasen <matthiasc src gnome org>]

commit 0d3df4fb1d56d33133e5c58c014cdf369b3f00b7
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Mar 14 01:23:09 2020 -0400

    Document gdk_toplevel_set_modal

 gdk/gdktoplevel.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/gdk/gdktoplevel.c b/gdk/gdktoplevel.c
index 1cfeb2c448..7544b673a2 100644
--- a/gdk/gdktoplevel.c
+++ b/gdk/gdktoplevel.c
@@ -352,6 +352,19 @@ gdk_toplevel_set_transient_for (GdkToplevel *toplevel,
   g_object_set (toplevel, "transient-for", parent, NULL);
 }
 
+/**
+ * gdk_toplevel_set_modal:
+ * @toplevel: A toplevel surface
+ * @modal: %TRUE if the surface is modal, %FALSE otherwise.
+ *
+ * The application can use this hint to tell the
+ * window manager that a certain surface has modal
+ * behaviour. The window manager can use this information
+ * to handle modal surfaces in a special way.
+ *
+ * You should only use this on surfaces for which you have
+ * previously called gdk_toplevel_set_transient_for().
+ */
 void
 gdk_toplevel_set_modal (GdkToplevel *toplevel,
                         gboolean     modal)