Improved man page for sde clean config

src/doc/man/sde-clean.txt

@@ -28,7 +28,8 @@ DISCRIMINATORS
 	no architectures specified, ccache files for all architectrues
 	will be removed.
 *config, cfg*::
-	Removed specified configurations (items).
+	Removed specified configurations (items). If no items specified
+	'default' will be used.
 *download*::
 	Removes old downloads from your old local download mirror. Only 
 	downloads still specified in .desc files will not be removed.