[4.1] cfc29b3 Document vxid VSL queries

[Martin Blix Grydeland]

commit cfc29b32a5c2839734e432a843d7390555c0add7
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Thu Feb 23 14:45:18 2017 +0100

    Document vxid VSL queries

diff --git a/doc/sphinx/reference/vsl-query.rst b/doc/sphinx/reference/vsl-query.rst
index 8f47156..a91d404 100644
--- a/doc/sphinx/reference/vsl-query.rst
+++ b/doc/sphinx/reference/vsl-query.rst
@@ -30,6 +30,8 @@ A query is run on a group of transactions. A query expression is true
 if there is a log record within the group that satisfies the
 condition. It is false only if none of the log records satisfies the
 condition. Query expressions can be combined using boolean functions.
+In addition to log records, it is possible to query transaction ids
+(vxid) in query.
 
 GROUPING
 ========
@@ -119,6 +121,16 @@ records. ::
 
   <record selection criteria> <operator> <operand>
 
+Additionally, a query expression can occur on the transaction
+itself rather than log records belonging to the transaction. ::
+
+  vxid <numerical operator> <integer>
+
+A ``vxid`` query allows you to directly target a specific transacion,
+whose id can be obtained from an ``X-Varnish`` HTTP header, the
+default "guru meditation" error page, or ``Begin`` and ``Link`` log
+records.
+
 Record selection criteria
 -------------------------
 
@@ -273,6 +285,10 @@ QUERY EXPRESSION EXAMPLES
 
     BerespStatus >= 500 or {2+}Timestamp:Process[2] > 1.
 
+* Log non-transactional errors. (Assumes raw grouping mode). ::
+
+    vxid == 0 and Error
+
 HISTORY
 =======