document setter function passing previous value (#961)

Co-authored-by: Atila Fassina <atilafassina@gmail.com>
Co-authored-by: Sarah <gerrardsarah@gmail.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

src/routes/concepts/signals.mdx

@@ -49,6 +49,14 @@ setCount(count() + 1);
 console.log(count()); // output: 1
 ```
 
+The setter function can also take a function that passes the previous value.
+
+```jsx
+setCount((prevCount) => prevCount + 1);
+
+console.log(count()); // output: 1
+```
+
 ## Reactivity
 
 Signals are reactive, which means that they automatically update when their value changes.