[gtk/matthiasc/for-master] docs: Mention subclassing and destroy in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 6a1cb3304b0abf3d20541c48f889c4bbc6fa776f
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Oct 20 18:26:38 2020 -0400

    docs: Mention subclassing and destroy in the migration guide
    
    Mention changes regarding subclassing and life-cycle
    handling in the migration guide.

 docs/reference/gtk/migrating-3to4.md | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 6ce96a8878..44f7d785b9 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -239,6 +239,31 @@ you get to the point that you actually build your application against
 GTK 4. Making it possible to prepare for these in GTK 3 would
 have been either impossible or impractical.
 
+### Larger changes
+
+Some of the larger themes of GTK 4 development are hard to cover in the form
+of checklist items, so we mention them separately up-front.
+
+#### Subclassing
+
+Compared to previous versions, GTK 4 emphasizes composition and delegation
+over subclassing. As a consequence, many widgets can no longer be subclassed.
+In most cases, you should look deriving your widget directly from GtkWidget
+and use complex widgets as child widgets instead of deriving from them.
+
+#### Life-cycle handling
+
+Widgets in GTK 4 are treated like any other objects - their parent widget
+holds a reference on them, and GTK holds a reference on toplevel windows.
+gtk_window_destroy() will drop the reference on the toplevel window, and
+cause the whole widget hierarchy to be finalized unless there are other
+references that keep widgets alive.
+
+The #GtkWidget::destroy signal is emitted when a widget is disposed, and
+therefore can no longer be used to break reference cycles. A typical sign
+of a reference cycle involving a toplevel window is when closing the window
+does not make the application quit.
+
 ### Stop using GdkScreen
 
 The GdkScreen object has been removed in GTK 4. Most of its APIs already