docs: add explanation for scoped slots with render functions (#2670)

Author: dbzx10299

src/guide/extras/render-function.md

@@ -577,6 +577,41 @@ JSX equivalent:
 
 Passing slots as functions allows them to be invoked lazily by the child component. This leads to the slot's dependencies being tracked by the child instead of the parent, which results in more accurate and efficient updates.
 
+### Scoped Slots
+
+To render a scoped slot in the parent component, a slot is passed to the child. Notice how the slot now has a parameter `text`. The slot will be called in the child component and the data from the child component will be passed up to the parent component.
+
+```js
+// parent component
+export default {
+  setup() {
+    return () => h(MyComp, null, {
+      default: ({ text }) => h('p', text)
+    })
+  }
+}
+```
+
+Remember to pass `null` so the slots will not be treated as props.
+
+```js
+// child component
+export default {
+  setup(props, { slots }) {
+    const text = ref('hi')
+    return () => h('div', null, slots.default({ text: text.value }))
+  }
+}
+```
+
+JSX equivalent:
+
+```jsx
+<MyComponent>{{
+  default: ({ text }) => <p>{ text }</p>  
+}}</MyComponent>
+```
+
 ### Built-in Components {#built-in-components}
 
 [Built-in components](/api/built-in-components) such as `<KeepAlive>`, `<Transition>`, `<TransitionGroup>`, `<Teleport>` and `<Suspense>` must be imported for use in render functions: