[18] Add start/useTransition docs (#4479)

* [18] Add start/useTransition docs

* Updates based on feedback

content/docs/hooks-reference.md

@@ -565,7 +565,43 @@ Memoizing the children tells React that it only needs to re-render them when `de
 const [isPending, startTransition] = useTransition();
 ```
 
-TODO: description
+Returns a stateful value for the pending state of the transition, and a function to start it.
+
+`startTransition` lets you mark updates in the provided callback as transitions:
+
+```js
+startTransition(() => {
+  setCount(count + 1);
+})
+```
+
+`isPending` indicates when a transition is active to show a pending state:
+
+```js
+function App() {
+  const [isPending, startTransition] = useTransition();
+  const [count, setCount] = useState(0);
+
+  function handleClick() {
+    startTransition(() => {
+      setCount(c => c + 1);
+    })
+  }
+
+  return (
+    <div>
+      {isPending && <Spinner />}
+      <button onClick={handleClick}>{count}</button>
+    </div>
+  );
+}
+```
+
+> Note:
+>
+> Updates in a transition yield to more urgent updates such as clicks.
+>
+> Updates in a transitions will not show a fallback for re-suspended content. This allows the user to continue interacting with the current content while rendering the update.
 
 ### `useId` {#useid}
 

content/docs/reference-react.md

@@ -383,9 +383,12 @@ a higher priority than neighboring boundaries. [Read more](https://github.com/re
 ```js
 React.startTransition(callback)
 ```
-
-TODO: description
+`React.startTransition` lets you mark updates inside the provided callback as transitions. This method is designed to be used when [`React.useTransition`](/docs/hooks-reference.html#usetransition) is not available.
 
 > Note:
-> 
-> TODO: useTransition
+>
+> Updates in a transition yield to more urgent updates such as clicks.
+>
+> Updates in a transitions will not show a fallback for re-suspended content, allowing the user to continue interacting while rendering the update.
+>
+> `React.startTransition` does not provide an `isPending` flag. To track the pending status of a transition see [`React.useTransition`](/docs/hooks-reference.html#usetransition).