[gtk/global-coords] Mention global coordinates in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit d47434bdb6da8df2603e6a23859a9d73c7e1f760
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed May 29 18:11:57 2019 +0000

    Mention global coordinates in the migration guide

 docs/reference/gtk/migrating-3to4.xml | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index a1d9dd08b5..1e3a9079eb 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -314,8 +314,15 @@
         doubles, and the _double variants have been removed. Update your
         code accordingly.
       </para>
+      <para>
+        Any APIs that deal with global (or root) coordinates have been
+        removed in GTK4, since not all backends support them. You should
+        replace your use of such APIs with surface-relative equivalents.
+        Examples of this are gdk_surfae_get_origin(), gdk_surface_move()
+        or gdk_event_get_root_coords().
+      </para>
     </section>
-
+ 
     <section>
       <title>Adapt to GdkKeymap API changes</title>
       <para>