Warner Bros. spelled out the details on the game's Steam community page. It's pretty straightforward, save for one exception: the Arkham Knight season pass is only eligible for a refund with the main game, you can't return the DLC separately. Not all versions of the game seem eligible for a refund, either -- users who purchased a retail copy of the game are reporting that the refund page won't load for them.

Users that choose not to (or aren't able to) take the refund are also being promised that Warner Bros. will continue to work on the game, but carefully notes that it may not be able to fix some issues. That's not promising. Unless you're one of the lucky few PC gamers for whom this title runs well, you should probably just get a refund.