[libadwaita/wip/exalm/dark: 6/6] doc: Mention default color scheme change in migration guide

[Alexander Mikhaylenko <alexm src gnome org>]

commit 082d6f13a202bddc61a0969e9f5ea56000b0d513
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 | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/doc/migrating-libhandy-1-4-to-libadwaita.md b/doc/migrating-libhandy-1-4-to-libadwaita.md
index 8c091abb..c5fde5ad 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -286,6 +286,14 @@ 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_PREFER_LIGHT` instead of
+`HDY_COLOR_SCHEME_FORCE_LIGHT`, following the system dark style preference by
+default. Make sure your application works with it, or otherwise set the
+`ADW_COLOR_SCHEME_FORCE_LIGHT` color scheme manually.
+
 ## Adapt to Stylesheet Changes
 
 Most widgets don't have a backdrop state anymore, and the following public