Update rendering-elements.md (#2804)

Link each mention of a React API in rendering-elements.md to the reference docs.

On this page replace every static occurrence of `ReactDOM.render()` by link to the reference docs [`ReactDOM.render()`](/docs/react-dom.html#render)

Is this in the right direction @sophiebits ? 🤔 

ref #2630

content/docs/rendering-elements.md

@@ -34,7 +34,7 @@ We call this a "root" DOM node because everything inside it will be managed by R
 
 Applications built with just React usually have a single root DOM node. If you are integrating React into an existing app, you may have as many isolated root DOM nodes as you like.
 
-To render a React element into a root DOM node, pass both to `ReactDOM.render()`:
+To render a React element into a root DOM node, pass both to [`ReactDOM.render()`](/docs/react-dom.html#render):
 
 `embed:rendering-elements/render-an-element.js`
 
@@ -46,19 +46,19 @@ It displays "Hello, world" on the page.
 
 React elements are [immutable](https://en.wikipedia.org/wiki/Immutable_object). Once you create an element, you can't change its children or attributes. An element is like a single frame in a movie: it represents the UI at a certain point in time.
 
-With our knowledge so far, the only way to update the UI is to create a new element, and pass it to `ReactDOM.render()`.
+With our knowledge so far, the only way to update the UI is to create a new element, and pass it to [`ReactDOM.render()`](/docs/react-dom.html#render).
 
 Consider this ticking clock example:
 
 `embed:rendering-elements/update-rendered-element.js`
 
 [](codepen://rendering-elements/update-rendered-element)
 
-It calls `ReactDOM.render()` every second from a [`setInterval()`](https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setInterval) callback.
+It calls [`ReactDOM.render()`](/docs/react-dom.html#render) every second from a [`setInterval()`](https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setInterval) callback.
 
 >**Note:**
 >
->In practice, most React apps only call `ReactDOM.render()` once. In the next sections we will learn how such code gets encapsulated into [stateful components](/docs/state-and-lifecycle.html).
+>In practice, most React apps only call [`ReactDOM.render()`](/docs/react-dom.html#render) once. In the next sections we will learn how such code gets encapsulated into [stateful components](/docs/state-and-lifecycle.html).
 >
 >We recommend that you don't skip topics because they build on each other.