PATCH: document beresp.http.header

[Federico G. Schwindt]

diff --git a/doc/sphinx/reference/vcl.rst b/doc/sphinx/reference/vcl.rst
index 1c1d8a2..1ad222f 100644
--- a/doc/sphinx/reference/vcl.rst
+++ b/doc/sphinx/reference/vcl.rst
@@ -719,6 +719,9 @@ beresp.do_gunzip
   Boolean. Unzip the object before storing it in the cache.  Defaults
   to false.
 
+beresp.http.header
+  The corresponding HTTP header.
+
 beresp.proto
   The HTTP protocol version used the backend replied with.