[gtk/gtk-4-4] docs: Mention popovers in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 16c3864e4a1205b23bdad56decd18bdd24a92dce
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Sep 28 15:47:06 2021 -0400

    docs: Mention popovers in the migration guide
    
    Things have changed around popovers, we should mention
    that.

 docs/reference/gtk/migrating-3to4.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 45b007549b..492e8628e4 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -1332,6 +1332,18 @@ pointer coordinates as inout arguments any more, but as normal in ones.
 
 See: [method@Gtk.TreeView.get_tooltip_context], [method@Gtk.IconView.get_tooltip_context]
 
+### Adapt to GtkPopover changes
+
+In GTK 3, a `GtkPopover` could be attached to any widget, using the `relative-to`
+property. This is no longer possible in GTK 4. The parent widget has to be aware
+of its popover children, and manage their size allocation. Therefore, only widgets
+with dedicated popover support can have them, such as [class@Gtk.MenuButton] or
+[class@Gtk.PopoverMenuBar].
+
+If you want to make a custom widget that has an attached popover, you need to call
+[method@Gtk.Popover.present] in your [vfunc@Gtk.Widget.size_allocate] vfunc, in order
+to update the positioning of the popover.
+
 ### Stop using GtkFileChooserButton
 
 The `GtkFileChooserButton` widget was removed, due to its shortcomings in