[gtk] docs: Mention workarea in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 7e355cbe1ed951b024f2f2b05823e9eab55de0dd
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jul 29 12:39:57 2020 -0400

    docs: Mention workarea in the migration guide

 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 0d3bf4ab21..f74f80f92a 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -774,6 +774,14 @@ Instead of a monitor number, #GdkMonitor is now used throughout.
 gdk_display_get_monitors() returns the list of monitors that can be queried
 or observed for monitors to pass to APIs like gtk_window_fullscreen_on_monitor().
 
+### Adapt to monitor API changes
+
+The gdk_monitor_get_workarea() API is gone. Individual backends can still
+provide this information, for example with gdk_x11_monitor_get_workarea().
+
+If you use this information, your code should check which backend is in
+use and then call the appropriate backend API.
+
 ### Adapt to cursor API changes
 
 Use the new gtk_widget_set_cursor() function to set cursors, instead of