[gtk/text] Mention GtkEditable and entries in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit f0717797e23d693559229a6f2085a80fafa8f353
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Feb 19 00:10:24 2019 -0500

    Mention GtkEditable and entries in the migration guide
    
    Not a very exhaustive treatment, but at least it gives
    some hints.

 docs/reference/gtk/migrating-3to4.xml | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index 53e49c77b2..af5a689a6d 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -601,6 +601,23 @@
         to pages. You can easily do that yourself.
       </para>
     </section>
+
+    <section>
+      <title>Adapt to changes in the API of GtkEntry and GtkSearchEntry</title>
+      <para>
+        The GtkEditable has been made more useful, and the core functionality of
+        GtkEntry has been broken out as a GtkText widget. GtkEntry, GtkSearchEntry,
+        GtkSpinButton and the new GtkPasswordEntry now use a GtkText widget internally
+        and implement GtkEditable. In particular, this means that it is no longer
+        possible to use GtkEntry API such as gtk_entry_grab_focus_without_selecting()
+        on a search entry.
+      </para>
+      <para>
+        Use GtkEditable API for editable functionality, and widget-specific APIs for
+        things that go beyond the common interface. For password entries, use
+        GtkPasswordEntry.
+      </para>
+    </section>
   </section>
 
 </chapter>