[libadwaita/wip/cdavis/stylesheet-selection-doc] doc: Mention that selection mode does nothing

[Christopher Davis <christopherdavis src gnome org>]

commit 4f779c83069cd48044a1043bd3367ee25f5b2224
Author: Christopher Davis <christopherdavis gnome org>
Date:   Thu Oct 14 10:17:24 2021 -0700

    doc: Mention that selection mode does nothing

 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 7351cf0e..1842d42b 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -346,6 +346,10 @@ If you were using `@theme_selected_bg_color` as a text color, use
 `@accent_color` instead to make sure the text is readable. You can also use the
 `.accent` style class to apply the correct color.
 
+The `.selection-mode` style class no longer recolors headerbars. Instead
+of using the old styling, rework your selection modes to follow
+[the new pattern for selection modes](TODO: Link to HIG). 
+
 #### Make sure your sidebars have borders
 
 The `.sidebar` style class doesn't provide a border by default. Instead, use a