[glib] Document new option of glib-genmarshal

[Matthias Clasen <matthiasc src gnome org>]

commit c382da4819ea0bd3a5f488106f24bc029c7b5c16
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Oct 12 15:56:04 2016 -0400

    Document new option of glib-genmarshal

 docs/reference/gobject/glib-genmarshal.xml |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gobject/glib-genmarshal.xml b/docs/reference/gobject/glib-genmarshal.xml
index 50a4251..5ddbc34 100644
--- a/docs/reference/gobject/glib-genmarshal.xml
+++ b/docs/reference/gobject/glib-genmarshal.xml
@@ -322,6 +322,13 @@ Print version and exit.
 </para></listitem>
 </varlistentry>
 
+<varlistentry>
+<term><option>--output=FILE</option></term>
+<listitem><para>
+Write output to FILE instead of stdout.
+</para></listitem>
+</varlistentry>
+
 </variablelist>
 </refsect1>