Add VCL design pattern about resp.status and modulus operator.

doc/sphinx/reference/dp_vcl_resp_status.rst

+.. _dp_vcl_resp_status:
+
+Using extra digits in resp.status 
+=================================
+
+In Varnish the ``.status`` variables can hold more than three
+digits, which is useful to send information to ``vcl_synth{}``
+about which error message to produce::
+
+    sub vcl_recv {
+       if ([...]) {
+           return(synth(12404));
+       }
+    }
+
+    sub vcl_synth {
+        if (resp.status == 12404) {
+            [...]       // this specific 404
+        } else if (resp.status % 1000 == 404) {
+            [...]       // all other 404's
+        }
+    }

doc/sphinx/reference/index.rst

@@ -23,6 +23,7 @@ VCL Design Patterns
 	:maxdepth: 1
 
 	dp_vcl_recv_hash.rst
+	dp_vcl_resp_status.rst
 
 Bundled VMODs
 -------------