[master] 262aa04 document vcl_synth

[Per Buer]

commit 262aa042fded0d0e46fac03032c9850116f4b702
Author: Per Buer <perbu at varnish-software.com>
Date:   Fri Oct 10 09:02:53 2014 +0200

    document vcl_synth

diff --git a/doc/sphinx/users-guide/vcl-built-in-subs.rst b/doc/sphinx/users-guide/vcl-built-in-subs.rst
index 276265c..48b19c9 100644
--- a/doc/sphinx/users-guide/vcl-built-in-subs.rst
+++ b/doc/sphinx/users-guide/vcl-built-in-subs.rst
@@ -229,6 +229,23 @@ with one of the following keywords:
     the number of retries is higher than *max_retries* Varnish emits a
     guru meditation error.
 
+vcl_synth
+~~~~~~~~~
+
+Called to deliver a synthetic object. A synthetic object is generated
+in VCL, not fetched from the backend. It is typically contructed using
+the synthetic() function.
+
+
+The subroutine may terminate with calling ``return()`` with one of the
+following keywords:
+
+  deliver 
+    Deliver the object. If the object has a positive TTL then the
+    object is also stored in cache.
+
+  restart
+    Restart processing the object.
 
 vcl_init
 ~~~~~~~~