[4.1] d243b4a Clarify VRT_delete_backend a bit

[Lasse Karstensen]

commit d243b4a1ebd117eef1dd1ade667be033ed88882c
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Fri Dec 4 15:09:02 2015 +0100

    Clarify VRT_delete_backend a bit

diff --git a/bin/varnishd/cache/cache_backend_cfg.c b/bin/varnishd/cache/cache_backend_cfg.c
index c734d80..ad96330 100644
--- a/bin/varnishd/cache/cache_backend_cfg.c
+++ b/bin/varnishd/cache/cache_backend_cfg.c
@@ -160,6 +160,9 @@ VRT_delete_backend(VRT_CTX, struct director **dp)
 	VTAILQ_REMOVE(&backends, be, list);
 	VTAILQ_INSERT_TAIL(&cool_backends, be, list);
 	Lck_Unlock(&be->mtx);
+
+	// NB. The backend is still usable for the ongoing transactions,
+	// this is why we don't bust the director's magic number.
 }
 
 /*---------------------------------------------------------------------