[gtk/ebassi/docs-for-master: 3/3] docs: Mention the ::damage-event signal in the migration guide

[Emmanuele Bassi <ebassi src gnome org>]

commit 71be3a8de1c7a1d4d9712e30db343289e48d7b95
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Jun 9 18:11:10 2021 +0100

    docs: Mention the ::damage-event signal in the migration guide
    
    There is no replacement for damage event tracking.

 docs/reference/gtk/migrating-3to4.md | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index f286fc72dc..0ffe95a8b5 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -200,6 +200,9 @@ on a one-by-one basis:
  - If you were using `::map-event` and `::unmap-event` to track a window
    being mapped, you should use property notification for the
    [property@Gdk.Surface:mapped] property instead.
+ - The `::damage-event` signal has no replacement, as the only consumer
+   of damage events were the offscreen GDK surfaces, which have no
+   replacement in GTK 4.x.
 
 ### Set a proper application ID