[gtk/wip/baedert/for-master: 29/29] migration guide: Mention button hierarchy changes

[Matthias Clasen <matthiasc src gnome org>]

commit cff211ea1f50a9dcb1617eb27939df8e9fac5a73
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Aug 30 22:25:56 2020 -0400

    migration guide: Mention button hierarchy changes

 docs/reference/gtk/migrating-3to4.md | 11 +++++++++++
 1 file changed, 11 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 665d5bcd8f..27e6ead12c 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -490,6 +490,17 @@ as property. GtkNotebook and GtkAssistant are similar.
 gtk4-builder-tool can help with this conversion, with the --3to4 option
 of the simplify command.
 
+### Adapt to button class hierarchy changes
+
+#GtkCheckButton is no longer derived from #GtkToggleButton. Call
+gtk_check_button_set_active() instead of gtk_toggle_button_set_active().
+
+#GtkRadioButton has been removed, and its grouping functionality has
+been added to #GtkCheckButton and #GtkToggleButton. Use grouped
+check buttons for traditional radio groups, and used grouped toggle
+buttons for view switchers. The new API to set up groups of buttons
+is gtk_check_button_set_group() and gtk_toggle_button_set_group().
+
 ### Adapt to GtkScrolledWindow API changes
 
 The constructor for GtkScrolledWindow no longer takes the adjustments