[clutter: 1/2] docs: Add missing settings.ini Environment group keys

[Emmanuele Bassi <ebassi src gnome org>]

commit c32fe995b60e1db30e6c1f6a83f095ab88c983cd
Author: Chris Mayo <aklhfex gmail com>
Date:   Fri Aug 30 20:06:48 2019 +0100

    docs: Add missing settings.ini Environment group keys
    
    Backends, Drivers and SyncToVblank are available but not documented in
    "Running Clutter".

 doc/reference/running-clutter.xml | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/doc/reference/running-clutter.xml b/doc/reference/running-clutter.xml
index ca50218b2..54f8ffd6c 100644
--- a/doc/reference/running-clutter.xml
+++ b/doc/reference/running-clutter.xml
@@ -338,6 +338,16 @@
         <title>Keys available for the Environment group</title>
 
         <variablelist>
+          <varlistentry>
+            <term>Backends</term>
+            <listitem><para>A string value, equivalent to setting
+            <code>CLUTTER_BACKEND</code>.</para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term>Drivers</term>
+            <listitem><para>A string value, equivalent to setting
+            <code>CLUTTER_DRIVER</code>.</para></listitem>
+          </varlistentry>
           <varlistentry>
             <term>ShowFps</term>
             <listitem><para>A boolean value, equivalent to setting
@@ -358,6 +368,11 @@
             <listitem><para>A boolean value, equivalent to setting
             <code>CLUTTER_ENABLE_ACCESSIBILITY</code>.</para></listitem>
           </varlistentry>
+          <varlistentry>
+            <term>SyncToVblank</term>
+            <listitem><para>A boolean value, equivalent to setting
+            <code>CLUTTER_VBLANK</code>.</para></listitem>
+          </varlistentry>
           <varlistentry>
             <term>DefaultFps</term>
             <listitem><para>An integer value, equivalent to setting