[libadwaita/wip/exalm/accent] doc: Mention the new accent colors in migration guide

[Alexander Mikhaylenko <alexm src gnome org>]

commit b0d84c927d79d5a337f564342de4d2bcd54832b2
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Mon Jun 14 18:21:53 2021 +0500

    doc: Mention the new accent colors in migration guide

 doc/migrating-libhandy-1-4-to-libadwaita.md | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/doc/migrating-libhandy-1-4-to-libadwaita.md b/doc/migrating-libhandy-1-4-to-libadwaita.md
index 0d3e58de..7f1b0672 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -251,3 +251,7 @@ colors have been removed:
 
 The `.list-button` style class has been renamed to the more accurate name
 `.outline`.
+
+The public colors `@theme_selected_bg_color` and `@theme_selected_fg_color` have
+been removed. Instead, use `@accent_color` and `@accent_text`. If you want to
+use `@theme_selected_bg_color` for foreground, use `@accent_color_text` instead.