Added undefined to in the list of possible return types of render function (#5275)

Co-authored-by: Shanmughapriyan S <priyanshan03@gmail.com>
Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>

content/docs/reference-react-component.md

@@ -125,7 +125,7 @@ When called, it should examine `this.props` and `this.state` and return one of t
 - **Arrays and fragments.** Let you return multiple elements from render. See the documentation on [fragments](/docs/fragments.html) for more details.
 - **Portals**. Let you render children into a different DOM subtree. See the documentation on [portals](/docs/portals.html) for more details.
 - **String and numbers.** These are rendered as text nodes in the DOM.
-- **Booleans or `null`**. Render nothing. (Mostly exists to support `return test && <Child />` pattern, where `test` is boolean.)
+- **Booleans or `null` or `undefined`**. Render nothing. (Mostly exists to support `return test && <Child />` pattern, where `test` is boolean).
 
 The `render()` function should be pure, meaning that it does not modify component state, it returns the same result each time it's invoked, and it does not directly interact with the browser.