[4.1] 8c95a2f Start documenting that VCL cool down should go fast

[Lasse Karstensen]

commit 8c95a2f2b5a3d9342c43ca9c088f916f1f899aa7
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Fri Dec 4 16:01:08 2015 +0100

    Start documenting that VCL cool down should go fast

diff --git a/doc/sphinx/reference/vmod.rst b/doc/sphinx/reference/vmod.rst
index ad6e03f..57fa09c 100644
--- a/doc/sphinx/reference/vmod.rst
+++ b/doc/sphinx/reference/vmod.rst
@@ -390,6 +390,10 @@ reference by calling ``VRT_ref_vcl`` when you receive a ``VCL_EVENT_WARM`` and
 later calling ``VRT_rel_vcl`` once the background job is over. Receiving a
 ``VCL_EVENT_COLD`` is your cue to terminate any background job bound to a VCL.
 
+In the case where properly releasing resources may take some time, you can
+opt for an asynchronous worker, either by spawning a thread and tracking it, or
+by using Varnish's worker pools.
+
 There is also a ``VCL_EVENT_USE`` event. Please note that this event is now
 deprecated and may be removed in a future release. A warm VCL should be ready
 to use so no additional task should be postponed at use time.