[gtk+] Update gtk-builder-tool docs

[Matthias Clasen <matthiasc src gnome org>]

commit 01626e0f4bd187059b67279ed6143b1f28575391
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Jan 29 09:42:18 2016 -0500

    Update gtk-builder-tool docs

 docs/reference/gtk/gtk-builder-tool.xml |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/gtk-builder-tool.xml b/docs/reference/gtk/gtk-builder-tool.xml
index f89c8e1..be8113b 100644
--- a/docs/reference/gtk/gtk-builder-tool.xml
+++ b/docs/reference/gtk/gtk-builder-tool.xml
@@ -62,6 +62,11 @@
     <term><option>enumerate</option></term>
       <listitem><para>Lists all the named objects that are created in the .ui file.</para></listitem>
     </varlistentry>
+    <varlistentry>
+    <term><option>preview</option> <arg choice="opt"><replaceable>ID</replaceable></arg></term>
+      <listitem><para>Preview the object with the given ID. If ID is not specified,
+                try to find a suitable object to preview.</para></listitem>
+    </varlistentry>
   </variablelist>
 </refsect1>