Clarify what useLatest is for

I was touching this file and noticed a chance to clean up this little TODO comment by swapping in our useLatest hook and giving it better documentation.

src/useLatest.ts

@@ -8,12 +8,13 @@ Please see LICENSE in the repository root for full details.
 import { type RefObject, useRef } from "react";
 
 export interface LatestRef<T> extends RefObject<T> {
-  current: T;
+  current: T; // Always defined, unlike RefObject["current"]
 }
 
 /**
  * React hook that returns a ref containing the value given on the latest
- * render.
+ * render. Useful for accessing the latest value of something in an effect or
+ * callback when you don't want reactivity.
  */
 export function useLatest<T>(value: T): LatestRef<T> {
   const ref = useRef<T>(value);