[Bug] xPDOQuery->select() incomplete parameter type hint in PHPDoc

[smaddock]

As $columns can be either a CSV or array, this should be either @param mixed $columns, @param array|string $columns or @param string[]|string $columns.

https://github.com/modxcms/xpdo/blob/c4f7bc6aa6a3f83b12615d07c171d3b97f40328d/src/xPDO/Om/xPDOQuery.php#L220