docs(ppr): Add note about canary for PPR (#70053)

## Why?

Experimental PPR does not work for Next.js v14. Technically, you can try
using v14 canary, but at that point, you should just be using v15 canary
because both requires React v19.

- Fixes #69791

---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>

docs/02-app/02-api-reference/05-next-config-js/ppr.mdx

@@ -83,7 +83,7 @@ export default function Page() {
 
 ### Enabling PPR (Version 14)
 
-For version 14, you can enable it by adding the [`ppr`](/docs/app/api-reference/next-config-js/ppr) option to your `next.config.js` file. This will apply to all routes in your application:
+For version 14, you can enable it by adding the [`ppr`](/docs/app/api-reference/next-config-js/ppr) option to your `next.config.js` file. You must also be on the canary version to enable PPR. This will apply to all routes in your application:
 
 ```ts filename="next.config.ts" switcher
 import type { NextConfig } from 'next'
@@ -108,7 +108,7 @@ const nextConfig = {
 module.exports = nextConfig
 ```
 
-| Version   | Changes                        |
-| --------- | ------------------------------ |
-| `v15.0.0` | `incremental` value introduced |
-| `v14.0.0` | experimental `ppr` introduced  |
+| Version   | Changes                                     |
+| --------- | ------------------------------------------- |
+| `v15.0.0` | experimental `incremental` value introduced |
+| `v14.0.0` | experimental `ppr` introduced               |