phpdoc fixes

framework/db/Query.php

@@ -338,7 +338,7 @@ class Query extends Component implements QueryInterface
 	 * Queries a scalar value by setting [[select]] first.
 	 * Restores the value of select to make this query reusable.
 	 * @param string|Expression $selectExpression
-	 * @param Connection $db
+	 * @param Connection|null $db
 	 * @return bool|string
 	 */
 	private function queryScalar($selectExpression, $db)

framework/db/QueryBuilder.php

@@ -645,7 +645,7 @@ class QueryBuilder extends \yii\base\Object
 	}
 
 	/**
-	 * @param string|array $joins
+	 * @param array $joins
 	 * @param array $params the binding parameters to be populated
 	 * @return string the JOIN clause built from [[Query::$join]].
 	 * @throws Exception if the $joins parameter is not in proper format