add some comments

manifests/systemd_override.pp

-# Override systemd's apache configiuration.
+# Override systemd's apache configuration.
+
+# $private_tmp: if set to true then systemd will force Apache to use a
+# "private" /tmp directory (that is, when Apache writes to /tmp/ it is in
+# fact writing to a special directory in /var/tmp that no other process
+# has access to). This is a problem for certreq since certreq creates a
+# file in /tmp and then calls an external executable to get NetDB access
+# information that needs to access this file. A private /tmp makes this
+# impossible. So we set it to false by default.
 
 class su_apache::systemd_override (
-  $private_tmp = true
+  $private_tmp = false
 ) {
 
   systemd::dropin_file { 'apache2.conf':