Enable psalm to detect return type of EasyDB::tryFlatTransaction()
Enable psalm to detect return type of EasyDB::tryFlatTransaction()

This has been bugging me for a while, but just came up again; adding additional docblocks to allow psalm to detect the appropriate return type based on the return type of the passed callback.

snippet:
```php
        $out = $this->db->tryFlatTransaction(
            /**
            * @return array<int, Event>
            */
            function(EasyDB $db) use ($now, $limit) : array {
```
paragonie/easydb,SignpostMarv/easydb