Adds PHPDoc to Logger::log

Frankly, I don't know. Reading `Monolog\Logger`, it will throw an exception [at some point](https://github.com/Seldaek/monolog/blob/master/src/Monolog/Logger.php#L384), if `$level` is not `string`.

But, thanks for the thorough research @cyrgit. Leave it as `mixed` since PSR spec defines it that way.