[libadwaita/wip/exalm/dark] doc: Mention default color scheme change in migration guide

[Alexander Mikhaylenko <alexm src gnome org>]

commit 78c2e8a50d455660e659783cde16453229f0cd1a
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Mon Sep 20 20:59:55 2021 +0500

    doc: Mention default color scheme change 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 fd0973c7..12a16f4f 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -283,6 +283,13 @@ and setter. It can be replaced by [property@Adw.Avatar:custom-image].
 The `hdy_avatar_draw_to_pixbuf_async()` function has been removed, use the
 regular [method@Adw.Avatar.draw_to_pixbuf] instead.
 
+### Adapt to [class@Adw.StyleManager] API Changes
+
+When used with the default style manager, `ADW_COLOR_SCHEME_DEFAULT` is now
+equivalent to `ADW_COLOR_SCHEME_LIGHT_DARK` instead of `HDY_COLOR_SCHEME_LIGHT`,
+following the dark style preference by default. Make sure your application works
+with it, or otherwise set the `ADW_COLOR_SCHEME_LIGHT` color scheme manually.
+
 ## Adapt to Stylesheet Changes
 
 Most widgets don't have a backdrop state anymore, and the following public