[4.1] feb2c2e document what the default backend is

[Lasse Karstensen]

commit feb2c2ebdfd16663fbb88a658abfc42238d7bff3
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Sat May 14 19:34:44 2016 +0200

    document what the default backend is

diff --git a/doc/sphinx/users-guide/vcl-backends.rst b/doc/sphinx/users-guide/vcl-backends.rst
index b44ba9c..5844891 100644
--- a/doc/sphinx/users-guide/vcl-backends.rst
+++ b/doc/sphinx/users-guide/vcl-backends.rst
@@ -74,6 +74,10 @@ really arbitrary data. You want to send mobile devices to a different
 backend? No problem. ``if (req.http.User-agent ~ /mobile/) ..`` should do the
 trick.
 
+Without an explicit backend selection, varnish will continue using
+the `default` backend. If there is no backend named `default`, the
+first backend found in the vcl will be used as the default backend.
+
 
 Backends and virtual hosts in Varnish
 -------------------------------------