API docs for useDeferredValue's initialValue (#6747)

* API docs for useDeferredValue's initialValue

Updates the API docs for `useDeferredValue` to include the
`initialValue` option, added in
facebook/react#27500.

This feature is slated for release in React 19.

* Add docs for onCaughtError and onUncaughtError (#6742)

* Add docs for onCaughtError and onUncaughtError

* Updates from feedback

* Add canary info, simplify a bit

---------

Co-authored-by: Ricky <[email protected]>

Author: acdlite

Diff for: src/content/reference/react/useDeferredValue.md

@@ -18,7 +18,7 @@ const deferredValue = useDeferredValue(value)
 
 ## Reference {/*reference*/}
 
-### `useDeferredValue(value)` {/*usedeferredvalue*/}
+### `useDeferredValue(value, initialValue?)` {/*usedeferredvalue*/}
 
 Call `useDeferredValue` at the top level of your component to get a deferred version of that value.
 
@@ -37,13 +37,23 @@ function SearchPage() {
 #### Parameters {/*parameters*/}
 
 * `value`: The value you want to defer. It can have any type.
+* <CanaryBadge title="This feature is only available in the Canary channel" /> **optional** `initialValue`: A value to use during the initial render of a component. If this option is omitted, `useDeferredValue` will not defer during the initial render, because there's no previous version of `value` that it can render instead.
+
 
 #### Returns {/*returns*/}
 
-During the initial render, the returned deferred value will be the same as the value you provided. During updates, React will first attempt a re-render with the old value (so it will return the old value), and then try another re-render in the background with the new value (so it will return the updated value). 
+- `currentValue`: During the initial render, the returned deferred value will be the same as the value you provided. During updates, React will first attempt a re-render with the old value (so it will return the old value), and then try another re-render in the background with the new value (so it will return the updated value).
+
+<Canary>
+
+In the latest React Canary versions, `useDeferredValue` returns the `initialValue` on initial render, and schedules a re-render in the background with the `value` returned.
+
+</Canary>
 
 #### Caveats {/*caveats*/}
 
+- When an update is inside a Transition, `useDeferredValue` always returns the new `value` and does not spawn a deferred render, since the update is already deferred.
+
 - The values you pass to `useDeferredValue` should either be primitive values (like strings and numbers) or objects created outside of rendering. If you create a new object during rendering and immediately pass it to `useDeferredValue`, it will be different on every render, causing unnecessary background re-renders.
 
 - When `useDeferredValue` receives a different value (compared with [`Object.is`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is)), in addition to the current render (when it still uses the previous value), it schedules a re-render in the background with the new value. The background re-render is interruptible: if there's another update to the `value`, React will restart the background re-render from scratch. For example, if the user is typing into an input faster than a chart receiving its deferred value can re-render, the chart will only re-render after the user stops typing.