[gtk/matthiasc/for-master] docs: Add gtk_widget_grab_default to migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 3c66c27caa94d9944dd592b0670f8f57c41d95f3
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jan 27 13:50:05 2021 -0500

    docs: Add gtk_widget_grab_default to migration guide
    
    Point out the replacement for this api.

 docs/reference/gtk/migrating-3to4.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 3d2fa1638b..597e77c78e 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -1079,6 +1079,11 @@ that wants to override the default handling, you can provide an
 implementation of the default.activate action in your widgets' action
 groups.
 
+### Stop using gtk_widget_grab_default()
+
+The function gtk_widget_grab_default() has been removed. If you need
+to mark a widget as default, use gtk_window_set_default_widget() directly.
+
 ### Stop setting ::has-default and ::has-focus in .ui files
 
 The special handling for the ::has-default and ::has-focus properties