docs: document story engine and PR2 playable loop

src/engine/game.ts

@@ -71,10 +71,7 @@ export function canUnlockAction(state: GameState, content: Content, actionId: st
 }
 
 export function isActionAvailable(state: GameState, content: Content, actionId: string): boolean {
-  return (
-    canAffordAction(state, content, actionId) &&
-    canUnlockAction(state, content, actionId)
-  );
+  return canAffordAction(state, content, actionId) && canUnlockAction(state, content, actionId);
 }
 
 function deductCosts(state: GameState, content: Content, actionId: string): void {