Docs: Updating guides on PPR and ISR (#81307)

The PR fixes issues in two different guides. 

### PPR
In PPR, the code for jsx and tsx versions should use the language
switcher. Currently, these are displayed below each other in the guide
which seems unintentional.

### ISR
This change fixes multiple issues:

**fixes in the example:**
the example currently does not match the text below (_Here's how this
example works:_). It says:
> If /blog/26 is requested, Next.js will generate and cache this page
on-demand

that is of course not correct because there are only 25 blog posts
returned from the API. No blog post data is returned for `/blog/26`,
making this example wrong and confusing.

By slicing the posts and supplying only 24 of 25 blog posts for static
generation during the build process, the guide becomes much more
explicit - the 25th blog post can be used for inspecting the workings of
the on-the-fly generation and caching. Turning `dynamicParams` to
`false` or `true`, one can now truly test this behaviour with the 25th
blog.

Second, the same example in Pages Router docs includes:
> return { paths, fallback: false }

In this case, again, this part below:

> If /blog/26 is requested, Next.js will generate and cache this page
on-demand

is incorrect. When setting `fallback` to `false`, the page is never
generated on-demand and returns 404 instead as there is no 26th blog
post in the fetched data. So the fix changes the value of `fallback` to
`blocking`, to mirror the App Router's `dynamicParams` value in the
guide. This makes is more consistent and, more importantly, reproducible
example.

**clarifying the on-demand section:**
The sections dedicated to **on-demand revalidation** do not currently
mention that revalidation works differently from the time-based one. One
may assume the first request after on-demand revalidation returns stale
data (just like time-based).

But that's not true, the next request after on-demand revalidation (with
Server Actions for example) generates the content on-the-fly and returns
fresh data right away. It wasn't clear to me so I tested it myself and
was surprised (and happy) that it is this smart. I think it's very
important to clarify this distinction in the guide.

-----

I hope this makes sense. I'm quite responsive these days and will change
it based on your feedback (if any). Thanks!

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---------

Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>

Author: MichalMoravik

docs/01-app/01-getting-started/06-partial-prerendering.mdx

@@ -142,7 +142,7 @@ export default function Layout({ children }: { children: React.ReactNode }) {
 }
 ```
 
-```jsx filename="/app/dashboard/layout.js" switcher
+```jsx filename="/app/dashboard/layout.jsx" switcher
 export const experimental_ppr = true
 
 export default function Layout({ children }) {

docs/01-app/02-guides/incremental-static-regeneration.mdx

@@ -35,11 +35,6 @@ interface Post {
 // request comes in, at most once every 60 seconds.
 export const revalidate = 60
 
-// We'll prerender only the params from `generateStaticParams` at build time.
-// If a request comes in for a path that hasn't been generated,
-// Next.js will server-render the page on-demand.
-export const dynamicParams = true // or false, to 404 on unknown paths
-
 export async function generateStaticParams() {
   const posts: Post[] = await fetch('https://api.vercel.app/blog').then((res) =>
     res.json()
@@ -67,16 +62,11 @@ export default async function Page({
 }
 ```
 
-```jsx filename="app/blog/[id]/page.js" switcher
+```jsx filename="app/blog/[id]/page.jsx" switcher
 // Next.js will invalidate the cache when a
 // request comes in, at most once every 60 seconds.
 export const revalidate = 60
 
-// We'll prerender only the params from `generateStaticParams` at build time.
-// If a request comes in for a path that hasn't been generated,
-// Next.js will server-render the page on-demand.
-export const dynamicParams = true // or false, to 404 on unknown paths
-
 export async function generateStaticParams() {
   const posts = await fetch('https://api.vercel.app/blog').then((res) =>
     res.json()
@@ -100,6 +90,15 @@ export default async function Page({ params }) {
 }
 ```
 
+Here's how this example works:
+
+1. During `next build`, all known blog posts are generated
+2. All requests made to these pages (e.g. `/blog/1`) are cached and instantaneous
+3. After 60 seconds has passed, the next request will still return the cached (now stale) page
+4. The cache is invalidated and a new version of the page begins generating in the background
+5. Once generated successfully, the next request will return the updated page and cache it for subsequent requests
+6. If `/blog/26` is requested, and it exists, the page will be generated on-demand. This behavior can be changed by using a different [dynamicParams](https://nextjs.org/docs/app/api-reference/file-conventions/route-segment-config#dynamicparams) value. However, if the post does not exist, then 404 is returned.
+
 </AppOnly>
 
 <PagesOnly>
@@ -125,10 +124,7 @@ export const getStaticPaths: GetStaticPaths = async () => {
     params: { id: String(post.id) },
   }))
 
-  // We'll prerender only these paths at build time.
-  // { fallback: 'blocking' } will server-render pages
-  // on-demand if the path doesn't exist.
-  return { paths, fallback: false }
+  return { paths, fallback: 'blocking' }
 }
 
 export const getStaticProps: GetStaticProps<Props> = async ({
@@ -167,9 +163,7 @@ export async function getStaticPaths() {
     params: { id: post.id },
   }))
 
-  // We'll prerender only these paths at build time.
-  // { fallback: false } means other routes should 404.
-  return { paths, fallback: false }
+  return { paths, fallback: 'blocking' }
 }
 
 export async function getStaticProps({ params }) {
@@ -195,16 +189,16 @@ export default function Page({ post }) {
 }
 ```
 
-</PagesOnly>
-
 Here's how this example works:
 
-1. During `next build`, all known blog posts are generated (there are 25 in this example)
+1. During `next build`, all known blog posts are generated
 2. All requests made to these pages (e.g. `/blog/1`) are cached and instantaneous
-3. After 60 seconds has passed, the next request will still show the cached (stale) page
+3. After 60 seconds has passed, the next request will still return the cached (now stale) page
 4. The cache is invalidated and a new version of the page begins generating in the background
-5. Once generated successfully, Next.js will display and cache the updated page
-6. If `/blog/26` is requested, Next.js will generate and cache this page on-demand
+5. Once generated successfully, the next request will return the updated page and cache it for subsequent requests
+6. If `/blog/26` is requested, and it exists, the page will be generated on-demand. This behavior can be changed by using a different [fallback](https://nextjs.org/docs/pages/api-reference/functions/get-static-paths#fallback-false) value. However, if the post does not exist, then 404 is returned.
+
+</PagesOnly>
 
 ## Reference
 
@@ -237,7 +231,7 @@ Here's how this example works:
 
 ### Time-based revalidation
 
-This fetches and displays a list of blog posts on `/blog`. After an hour, the cache for this page is invalidated on the next visit to the page. Then, in the background, a new version of the page is generated with the latest blog posts.
+This fetches and displays a list of blog posts on /blog. After an hour has passed, the next visitor will still receive the cached (stale) version of the page immediately for a fast response. Simultaneously, Next.js triggers regeneration of a fresh version in the background. Once the new version is successfully generated, it replaces the cached version, and subsequent visitors will receive the updated content.
 
 ```tsx filename="app/blog/page.tsx" switcher
 interface Post {
@@ -289,7 +283,7 @@ We recommend setting a high revalidation time. For instance, 1 hour instead of 1
 
 For a more precise method of revalidation, invalidate pages on-demand with the `revalidatePath` function.
 
-For example, this Server Action would get called after adding a new post. Regardless of how you retrieve your data in your Server Component, either using `fetch` or connecting to a database, this will clear the cache for the entire route and allow the Server Component to fetch fresh data.
+For example, this Server Action would get called after adding a new post. Regardless of how you retrieve your data in your Server Component, either using `fetch` or connecting to a database, this will immediately clear the cache for the entire route. Unlike time-based revalidation, the next request after on-demand revalidation won't return stale data but will serve the updated version right away and cache it for subsequent requests.
 
 ```ts filename="app/actions.ts" switcher
 'use server'