[gtk/stackpage: 5/11] Document --3to4

[Matthias Clasen <matthiasc src gnome org>]

commit 1fb3febacc293da9d36321c31aa4c099f59ace15
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Feb 7 11:57:09 2019 -0500

    Document --3to4

 docs/reference/gtk/gtk4-builder-tool.xml | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/docs/reference/gtk/gtk4-builder-tool.xml b/docs/reference/gtk/gtk4-builder-tool.xml
index e9f497599e..992f2d9db1 100644
--- a/docs/reference/gtk/gtk4-builder-tool.xml
+++ b/docs/reference/gtk/gtk4-builder-tool.xml
@@ -79,6 +79,10 @@
     <term><option>--replace</option></term>
       <listitem><para>Write the content back to the .ui file instead of stdout.</para></listitem>
     </varlistentry>
+    <varlistentry>
+    <term><option>--3to4</option></term>
+      <listitem><para>Transform a GTK 3 ui file to GTK 4</para></listitem>
+    </varlistentry>
   </variablelist>
 </refsect1>