Update the docs for Switchable Runtime (vercel#36015)

As per the feedback in vercel#35952.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

docs/advanced-features/react-18/switchable-runtime.md

@@ -1,6 +1,6 @@
 # Switchable Runtime (Alpha)
 
-By default, Next.js uses Node.js as the runtime for page rendering, including pre-rendering, server-side rendering.
+By default, Next.js uses Node.js as the runtime for page rendering, including pre-rendering and server-side rendering.
 
 If you have [React 18](/docs/advanced-features/react-18/overview) installed, there is a new experimental feature that lets you switch the page runtime between Node.js and the [Edge Runtime](/docs/api-reference/edge-runtime). Changing the runtime affects [SSR streaming](/docs/advanced-features/react-18/streaming) and [Server Components](/docs/advanced-features/react-18/server-components) features, as well.
 
@@ -19,7 +19,7 @@ module.exports = {
 
 This option determines which runtime should be used as the default rendering runtime for all pages.
 
-## Per-page Runtime Option
+## Page Runtime Option
 
 On each page, you can optionally export a `runtime` config set to either `'nodejs'` or `'edge'`:
 
@@ -32,3 +32,5 @@ export const config = {
 When both the per-page runtime and global runtime are set, the per-page runtime overrides the global runtime. If the per-page runtime is _not_ set, the global runtime option will be used.
 
 You can refer to the [Switchable Next.js Runtime RFC](https://github.com/vercel/next.js/discussions/34179) for more information.
+
+**Note:** The page runtime option is not supported in [API Routes](/docs/api-routes/introduction.md) currently.