docs: explain signal object and array updates (#1627)

docs: explain signal object updates

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: dongyikuan919

src/routes/(0)concepts/(2)signals.mdx

@@ -72,6 +72,17 @@ setCount((prevCount) => prevCount + 1);
 console.log(count()); // output: 1
 ```
 
+When a signal contains an object or array, mutating it directly does not notify subscribers because the setter was not called.
+Use the setter to create a new object or array instead:
+
+```jsx
+const [items, setItems] = createSignal(["apple", "banana"]);
+
+setItems((currentItems) => [...currentItems, "cherry"]);
+```
+
+For state that requires frequent nested updates, consider using a [store](/concepts/stores) instead.
+
 ## Reactivity
 
 Signals are reactive, which means that they automatically update when their value changes.