Update docs for image lazyRoot prop

docs/api-reference/next/image.md

@@ -220,11 +220,53 @@ You can also [generate a solid color Data URL](https://png-pixel.com) to match t
 
 A string (with similar syntax to the margin property) that acts as the bounding box used to detect the intersection of the viewport with the image and trigger lazy [loading](#loading). Defaults to `"200px"`.
 
+If the image is nested in a scrollable parent element other than the root document, you will also need to assign the [lazyRoot](#lazyroot) prop.
+
 [Learn more](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/rootMargin)
 
 ### lazyRoot
 
-A React [Ref](https://reactjs.org/docs/refs-and-the-dom.html) pointing to the Element which the [lazyBoundary](#lazyBoundary) calculates for the Intersection detection. Defaults to `null`, referring to the document viewport.
+A React [Ref](https://reactjs.org/docs/refs-and-the-dom.html) pointing to the scrollable parent element. Defaults to `null` (the document viewport).
+
+The Ref must point to a DOM element or a React component that [forwards the Ref](https://reactjs.org/docs/forwarding-refs.html) to the underlying DOM element.
+
+**Example pointing to a DOM element**
+
+```jsx
+import Image from 'next/image'
+import React from 'react'
+
+const lazyRoot = React.useRef(null)
+
+const Example = () => (
+  <div ref={lazyRoot} style={{ overflowX: 'scroll', width: '500px' }}>
+    <Image lazyRoot={lazyRoot} src="/one.jpg" width="500" height="500" />
+    <Image lazyRoot={lazyRoot} src="/two.jpg" width="500" height="500" />
+  </div>
+)
+```
+
+**Example pointing to a React component**
+
+```jsx
+import Image from 'next/image'
+import React from 'react'
+
+const Container = React.forwardRef((props, ref) =>
+  <div ref={ref} style={{ overflowX: 'scroll', width: '500px' }}>
+    {props.children}
+  </div>
+})
+
+const Example = () => {
+  const lazyRoot = React.useRef(null)
+
+  return (<Container ref={lazyRoot}>
+    <Image lazyRoot={lazyRoot} src="/one.jpg" width="500" height="500" />
+    <Image lazyRoot={lazyRoot} src="/two.jpg" width="500" height="500" />
+  </Container>)
+}
+```
 
 [Learn more](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/root)