Document grace

Thanks to perbu for suggested documentation
Fixes 355


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3402 d4fa192b-c00b-0410-8231-f00ffab90ce4

man/vcl.7so

@@ -186,6 +186,24 @@ if (client.ip ~ local) {
     pipe;
 }
 .Ed
+.Ss Grace 
+If the backend takes a long time to generate an object there is a risk
+of a thread pile up.
+In order to prevent this you can enable grace.
+This allows varnish to serve an expired version of the object while a
+fresh object is being generated by the backend.
+.Pp
+The following vcl code will make Varnish serve expired objects.
+All object will be kept up to two minutes past their expiration time
+or a fresh object is generated.
+.Bd -literal -offset 4n
+sub vcl_recv {
+    set req.grace = 2m;
+}
+sub vcl_fetch {
+    set obj.grace = 2m;
+}
+.Ed
 .Ss Functions
 The following built-in functions are available:
 .Bl -tag -width indent