[ci skip] DocBlocks for DB drivers' utility classes

Partially fixes issue #1295.
diff --git a/system/database/DB_utility.php b/system/database/DB_utility.php
index 3bd77c8..31102c0 100644
--- a/system/database/DB_utility.php
+++ b/system/database/DB_utility.php
@@ -45,23 +45,30 @@
 	// --------------------------------------------------------------------
 
 	/**
+	 * List databases statement
+	 *
+	 * @var	string
+	 */
+	$_list_databases		= FALSE;
+
+	/**
 	 * OPTIMIZE TABLE statement
 	 *
-	 * @var	string|bool
+	 * @var	string
 	 */
 	protected $_optimize_table	= FALSE;
 
 	/**
 	 * REPAIR TABLE statement
 	 *
-	 * @var	string|bool
+	 * @var	string
 	 */
 	protected $_repair_table	= FALSE;
 
 	// --------------------------------------------------------------------
 
 	/**
-	 * Constructor
+	 * Class constructor
 	 *
 	 * @return	void
 	 */