[master] d86f1850b Clarify the comment

[Dridi Boukelmoune]

commit d86f1850bc6ebabd61ef7274e2ff06784be0669f
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Wed Apr 10 13:21:49 2019 +0200

    Clarify the comment
    
    This is the intent that was not obvious to me when I first reviewed this
    change.

diff --git a/bin/varnishd/cache/cache_hash.c b/bin/varnishd/cache/cache_hash.c
index b108a3062..5576ca6fd 100644
--- a/bin/varnishd/cache/cache_hash.c
+++ b/bin/varnishd/cache/cache_hash.c
@@ -778,8 +778,8 @@ HSH_Cancel(struct worker *wrk, struct objcore *oc, struct boc *boc)
 		return;
 
 	/*
-	 * the caller-provided boc is optional, if they do not have one, we grab
-	 * a ref
+	 * NB: we use two distinct variables to only release the reference if
+	 * we had to acquire one. The caller-provided boc is optional.
 	 */
 	if (boc == NULL)
 		bocref = boc = HSH_RefBoc(oc);