note a regression in varnish-cache master

src/tests/usage.vtc

@@ -249,6 +249,8 @@ logexpect l2 -wait
 varnish v2 -stop
 
 # hmac constructor fails if the key is NULL
+# broken in varnish-cache master
+# ref: https://github.com/varnishcache/varnish-cache/pull/2941
 varnish v1 -errvcl {vmod blobdigest error: key is NULL in fail constructor} {
 	import blobdigest from "${vmod_topbuild}/src/.libs/libvmod_blobdigest.so";
 	import blob;