[libadwaita] doc: Mention Adw.ViewStack in migration guide

[Adrien Plazas <aplazas src gnome org>]

commit eaa662117e2c1946be3ebd87369ecebe902c845f
Author: CodedOre <CodedOre>
Date:   Mon Jun 28 17:16:44 2021 +0200

    doc: Mention Adw.ViewStack in migration guide

 doc/migrating-libhandy-1-4-to-libadwaita.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/doc/migrating-libhandy-1-4-to-libadwaita.md b/doc/migrating-libhandy-1-4-to-libadwaita.md
index e972df07..999ae0ba 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -227,6 +227,13 @@ page objects, similarly to [class@Gtk.Stack]. For example,
 [method@Adw.SqueezerPage.set_enabled] should be used to replace
 `hdy_squeezer_set_child_enabled()`.
 
+### Adapt to view switcher API Changes
+
+[class@Adw.ViewSwitcher], [class@Adw.ViewSwitcherBar] and
+[class@Adw.ViewSwitcherTitle] now use [class@Adw.ViewStack] instead of
+[class@Gtk.Stack]. For the most part, the new widget should be a drop-in
+replacement.
+
 ### Adapt to [class@Adw.Avatar] API Changes
 
 The `HdyAvatar:loadable-icon` property has been removed along with its getter