Docs: Enhance and correct selector memoization example in Essentials Part 6 (fixes #4820)

Author: synwix

docs/tutorials/essentials/part-6-performance-normalization.md

@@ -901,14 +901,20 @@ selectPostsByUser(state1, 'user2')
 
 dispatch(reactionAdded())
 const state2 = getState()
-// Output selector does not run, because `posts` and `userId` are the same
+// Output selector runs, because the `posts` has changed
 selectPostsByUser(state2, 'user2')
 
 // Add some more posts
 dispatch(addNewPost())
 const state3 = getState()
 // Output selector runs, because `posts` has changed
 selectPostsByUser(state3, 'user2')
+
+// Fetch notifications
+dispatch(fetchNotifications())
+const state4 = getState()
+// Output selector does not run, because `posts` and `userId` are the same
+selectPostsByUser(state4, 'user2')
 ```
 
 Now that we've memoized `selectPostsByUser`, we can try repeating the React profiler with `<UserPage>` open while fetching notifications. This time we should see that `<UserPage>` doesn't re-render: