Better doc of tmpfiles $ensure parameter.

Clarify that the $ensure parameter to systemd::tmpfiles only affects
the config file in /etc/tmpfiles.d itself, not the file or directories
specified in that config file.

manifests/tmpfiles.pp

@@ -103,6 +103,11 @@ define systemd::tmpfiles(
 
 	# One of 'present' or 'absent'.
 	#
+	# This only controls if the config file in /etc/tmpfiles.d should
+	# be present or removed.  In particular, setting this to 'absent'
+	# will only remove the config file itself, but won't remove the
+	# files/directories specified by the config file.
+	#
 	$ensure = 'present',
 )
 {