[gtk+] docs: gtk/running: Document the GTK_CSD env var

[Daniel Boles <dboles src gnome org>]

commit d6e4ce127a4e9d643eab64cbfc8db499942fd7f4
Author: Daniel Boles <dboles src gnome org>
Date:   Sun Aug 6 17:42:17 2017 +0100

    docs: gtk/running: Document the GTK_CSD env var
    
    It was not documented before.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=778791#c1

 docs/reference/gtk/running.sgml |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/running.sgml b/docs/reference/gtk/running.sgml
index 1d8048d..0794064 100644
--- a/docs/reference/gtk/running.sgml
+++ b/docs/reference/gtk/running.sgml
@@ -515,6 +515,21 @@ nevertheless.
 </formalpara>
 
 <formalpara>
+  <title><envar>GTK_CSD</envar></title>
+
+  <para>
+    The default value of this environment variable is 1. If changed to 0, this
+    disables the default use of client-side decorations on GTK+ windows, thus
+    making the window manager responsible for drawing the decorations of
+    windows that do not have a custom titlebar widget.
+  </para>
+  <para>
+    CSD is always used for windows with a custom titlebar widget set, as the WM
+    should not draw another titlebar or other decorations around the custom one.
+  </para>
+</formalpara>
+
+<formalpara>
   <title><envar>XDG_DATA_HOME</envar>, <envar>XDG_DATA_DIRS</envar></title>
 
   <para>