[3.0] efe5068 Document req.grace=0s strangeness

[Lasse Karstensen]

commit efe50681b6a5a1e113587acafbc14394386c34eb
Author: Lasse Karstensen <lkarsten at varnish-software.com>
Date:   Wed Mar 12 14:56:19 2014 +0100

    Document req.grace=0s strangeness
    
    Document that our use of 0s as a magic constant had the side
    effect of enabling grace if you tried to disable it in VCL, and
    how to get it to work.

diff --git a/doc/sphinx/reference/vcl.rst b/doc/sphinx/reference/vcl.rst
index 76aae84..ffd7c0c 100644
--- a/doc/sphinx/reference/vcl.rst
+++ b/doc/sphinx/reference/vcl.rst
@@ -760,6 +760,11 @@ req.esi_level
 req.grace
   Set to a period to enable grace.
 
+  Known limitation in 3.0: Disabling grace by setting req.grace or beresp.grace
+  to 0s does not have the desired effect, but will rather set the grace time to
+  the value of default_grace. To disable grace for a request, either set
+  parameter default_grace = 0s or set req.grace = 0.000001s in VCL.
+
 req.xid
   Unique ID of this request.