[gtk/matthiasc/for-master] docs: Add some more to the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit d7794bf60825e1e2d9c32b491195bd58b93adf52
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Oct 21 14:30:17 2020 -0400

    docs: Add some more to the migration guide
    
    Expand the section on life-cycle handling with some
    more details.

 docs/reference/gtk/migrating-3to4.md | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 44f7d785b9..7a70e21bc0 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -264,6 +264,14 @@ 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.
 
+A good rule to follow is: If you set a widget pointer with
+gtk_widget_class_bind_template_child() in class_init(), you need to
+unparent it in dispose(). The slight complication here is that you need
+to respect the widget hierarchy while doing so. Ie if you set both `field1`
+and `field2`, but `field1` is an ancestor of `field2`, then you only need
+to unparent `field1` — doing so will remove the the entire subtree below
+`field1`, including `field2`.
+
 ### Stop using GdkScreen
 
 The GdkScreen object has been removed in GTK 4. Most of its APIs already