Explain limitations of useSyncExternalStore with concurrency (#6339)

* Explain limitations of useSyncExternalStore with concurrency

Doesn't seem like we have this noted anywhere.

* Pull out example to code block

Author: sophiebits

src/content/reference/react/useSyncExternalStore.md

@@ -57,6 +57,26 @@ The current snapshot of the store which you can use in your rendering logic.
 
 * If a different `subscribe` function is passed during a re-render, React will re-subscribe to the store using the newly passed `subscribe` function. You can prevent this by declaring `subscribe` outside the component.
 
+* If the store is mutated during a [non-blocking transition update](/reference/react/useTransition), React will fall back to performing that update as blocking. Specifically, React will call `getSnapshot` a second time just before applying changes to the DOM. If it returns a different value than when it was called originally, React will restart the transition update from scratch, this time applying it as a blocking update, to ensure that every component on screen is reflecting the same version of the store.
+
+* It's not recommended to _suspend_ a render based on a store value returned by `useSyncExternalStore`. The reason is that mutations to the external store cannot be [marked as non-blocking transition updates](/reference/react/useTransition), so they will trigger the nearest [`Suspense` fallback](/reference/react/Suspense), replacing already-rendered content on screen with a loading spinner, which typically makes a poor UX.
+
+  For example, the following are discouraged:
+
+  ```js
+  const LazyProductDetailPage = lazy(() => import('./ProductDetailPage.js'));
+
+  function ShoppingApp() {
+    const selectedProductId = useSyncExternalStore(...);
+
+    // ❌ Calling `use` with a Promise dependent on `selectedProductId`
+    const data = use(fetchItem(selectedProductId))
+
+    // ❌ Conditionally rendering a lazy component based on `selectedProductId`
+    return selectedProductId != null ? <LazyProductDetailPage /> : <FeaturedProducts />;
+  }
+  ```
+
 ---
 
 ## Usage {/*usage*/}
@@ -425,4 +445,4 @@ function ChatIndicator({ userId }) {
 
   // ...
 }
-```
+```