doc: Tell to stop using hdy_init()

Let the users know that it has been removed and isn't needed anymore.

Fixes Librem5/libhandy#89.

doc/hdy-migrating-0-0-to-1.xml

@@ -70,6 +70,16 @@
       either impossible or impractical.
     </para>
 
+    <refsect3>
+      <title>hdy_init has been removed</title>
+      <para>
+        hdy_init() has been removed.
+        Instead, Handy 1 comes with a library constructor which will
+        automatically initialize it when linked.
+        You should just remove calls to hdy_init().
+      </para>
+    </refsect3>
+
   </refsect2>
 
 </refentry>