[4.1] c72f24a document the default probe

[Lasse Karstensen]

commit c72f24ad217075ca57c3fc4f06a95c8568f88ac2
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Wed May 11 16:14:17 2016 +0200

    document the default probe

diff --git a/doc/sphinx/reference/vcl.rst b/doc/sphinx/reference/vcl.rst
index c982cb9..553fd72 100644
--- a/doc/sphinx/reference/vcl.rst
+++ b/doc/sphinx/reference/vcl.rst
@@ -244,6 +244,9 @@ the backend as down it they fail. A probe is defined as this::
         .attribute = "value";
     }
 
+The probe named `default` is special and will be used for all backends
+which do not explicitly reference a probe.
+
 There are no mandatory options. These are the options you can set:
 
   url