[gtk/gesture-click] Mention event controller API in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 200373f4357d2351f3140484db6c5d6e38d2a267
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed May 29 14:17:07 2019 -0400

    Mention event controller API in the migration guide

 docs/reference/gtk/migrating-3to4.xml | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index a1d9dd08b5..d61dc892b7 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -324,6 +324,21 @@
       </para>
     </section>
 
+    <section>
+      <title>Adapt to event controller API changes</title>
+      <para>
+        A few changes to the event controller and #GtkGesture APIs
+        did not make it back to GTK3, and have to be taken into account
+        when moving to GTK4. One is that the
+        #GtkEventControllerMotion::enter,
+        #GtkEventControllerMotion::leave,
+        #GtkEventControllerKey::focus-in and
+        #GtkEventControllerKey::focus-out signals
+        have gained new arguments. Another is that #GtkGestureMultiPress
+        has been renamed to #GtkGestureClick.
+      </para>
+    </section>
+
     <section>
       <title>Stop using GtkEventBox</title>
       <para>