[master] 27aa567 update VCL_trace usage example

[Nils Goroll]

commit 27aa56736015ac021194f106e91985588a5b9d2c
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Fri Jun 30 11:15:07 2017 +0200

    update VCL_trace usage example

diff --git a/doc/sphinx/installation/bugs.rst b/doc/sphinx/installation/bugs.rst
index 091b7c7..2305c1c 100644
--- a/doc/sphinx/installation/bugs.rst
+++ b/doc/sphinx/installation/bugs.rst
@@ -150,9 +150,9 @@ than what you intended it to do. If it sounds like a bug that would
 have tripped up everybody else, take a moment to read through your
 VCL and see if it really does what you think it does.
 
-You can also try setting the ``vcl_trace`` parameter, that will generate log
-records with like and character number for each statement executed in your VCL
-program.
+You can also try setting the ``vsl_mask=+VCL_trace`` parameter (or use
+``varnishadm param.set vsl_mask +VCL_trace`` on a running instance),
+that will generate log records with like and character number for each
+statement executed in your VCL program.
 
 .. XXX:Example of the command perhaps? benc
-