[gtk: 3/4] docs: Document GtkBuildable changes in migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit e4d4b50d6de6324f5566cad9061ac7470fa64125
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Sep 25 22:54:52 2020 +0200

    docs: Document GtkBuildable changes in migration guide
    
    https://gitlab.gnome.org/GNOME/gtk/-/issues/3191

 docs/reference/gtk/migrating-3to4.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 59800003b5..bae17df96f 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -1041,6 +1041,13 @@ You can replace calls to gtk_dialog_run() by specifying that the
 %GTK_DIALOG_MODAL flag, and connecting to the #GtkDialog::response
 signal.
 
+### Stop using GtkBuildable API
+
+All the GtkBuildable API was made private, except for the
+getter function to retrieve the buildable ID. If you are
+using gtk_buildable_get_name() you should replace it with
+gtk_buildable_get_buildable_id().
+
 ## Changes to consider after the switch
 
 GTK 4 has a number of new features that you may want to take