mention the url to the issue about the services' init script

this way we can follow up on the situation, put pressure on the
community to get this fixed, and remove our hack when it's fixed.

manifests/disable/debian.pp

@@ -1,5 +1,9 @@
 # debian has some issues with absent
-# init scripts
+# init scripts.
+# It's a bug in debian's provider that should be fixed in puppet, but in the
+# meantime we need this hack.
+#
+# see: https://projects.puppetlabs.com/issues/9381
 class postfix::disable::debian inherits postfix::disable::base {
   Service['postfix']{
     hasstatus => false,