[gtk+] Document the new GDK_VULKAN_DEVICE envvar

[Matthias Clasen <matthiasc src gnome org>]

commit 6cab57d7edac3f377003d3a19641bb05693cac33
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jan 10 00:14:10 2018 -0500

    Document the new GDK_VULKAN_DEVICE envvar
    
    Best to document them while they're fresh.

 docs/reference/gtk/running.sgml |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/running.sgml b/docs/reference/gtk/running.sgml
index b717d71..827f7b7 100644
--- a/docs/reference/gtk/running.sgml
+++ b/docs/reference/gtk/running.sgml
@@ -471,7 +471,7 @@ nevertheless.
       <listitem><para>Disable Vulkan support</para></listitem>
     </varlistentry>
     <varlistentry>
-      <term>validate </term>
+      <term>validate</term>
       <listitem><para>Enable the Vulkan validation layer</para></listitem>
     </varlistentry>
   </variablelist>
@@ -482,6 +482,17 @@ nevertheless.
 </formalpara>
 
 <formalpara>
+  <title><envar>GDK_VULKAN_DEVICE</envar></title>
+
+  <para>
+    This variable can be set to the index of a Vulkan device to override the
+    default selection of the device that is used for Vulkan rendering.
+    The special value <literal>list</literal> can be used to obtain a list
+    of all Vulkan devices.
+  </para>
+</formalpara>
+
+<formalpara>
   <title><envar>GTK_CSD</envar></title>
 
   <para>