[ci skip] DocBlocks for DB drivers' driver classes

Partially fixes issue #1295.
diff --git a/system/database/DB_utility.php b/system/database/DB_utility.php
index 31102c0..f7bef6a 100644
--- a/system/database/DB_utility.php
+++ b/system/database/DB_utility.php
@@ -49,7 +49,7 @@
 	 *
 	 * @var	string
 	 */
-	$_list_databases		= FALSE;
+	protected $_list_databases		= FALSE;
 
 	/**
 	 * OPTIMIZE TABLE statement