[gtk: 1/2] docs: Mention GtkImage and GtkPicture changes

[Matthias Clasen <matthiasc src gnome org>]

commit 484c0fdd15704edd3a12e2faea3dae3f7f6cab5a
Author: Jakub Kulík <kulikjak gmail com>
Date:   Fri Dec 17 00:32:08 2021 +0000

    docs: Mention GtkImage and GtkPicture changes

 docs/reference/gtk/migrating-3to4.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 068b97212a..c45098d777 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -1276,6 +1276,15 @@ is provided in the form of a `GtkIconPaintable` (this can be checked with
 [method@Gtk.IconPaintable.is_symbolic]), you have to call
 [method@Gtk.IconPaintable.get_icon_name] and set the icon name on a `GtkImage`.
 
+### Adapt to GtkImage changes
+`GtkPicture`'s behaviour was "split out" of `GtkImage` as the latter was covering
+too many use cases; if you're loading an icon, [class@Gtk.Image] in GTK3 and GTK4 are
+perfectly equivalent. If you are loading a more complex image asset, like a picture
+or a thumbnail, then [class@Gtk.Picture] is the appropriate widget.
+
+One noteworthy distinction is that while `GtkImage` has its size computed by
+GTK, `GtkPicture` lets you decide about the size.
+
 ### Update to GtkFileChooser API changes
 
 `GtkFileChooser` moved to a GFile-based API. If you need to convert a path