Improve documentation of systemd::daemon_reload.

Mention that systemd::unit and systemd::unit_options automatically
includes and notifies systemd::daemon_reload, so users don't have to
do that themselves.

manifests/daemon_reload.pp

@@ -14,6 +14,10 @@
  *             ensure => absent,
  *             notify => Class['systemd::daemon_reload'];
  *     }
+ *
+ * The systemd::unit and systemd::unit_options definitions includes
+ * and notifies this class automatically, so users do not need to do
+ * that themselves.
  */
 class systemd::daemon_reload
 {