docs(cache): add a note for serverless environment

Author: atinux

docs/1.guide/6.cache.md

@@ -69,6 +69,38 @@ The stars will be cached in development inside **.nitro/cache/functions/ghStars/
 You can also use the `cachedFunction` method as alias of `defineCachedFunction`.
 ::
 
+### Serverless environment
+
+In a serverless environment, the instance is destroyed after each request. Nitro uses `event.waitUntil` to keep the instance alive while the cache is being updated while the response is sent to the client.
+
+To make sure your cached functions are working as expected in a serverless environment, you should always give the `event` as first argument to the function.
+
+::code-group
+```ts [utils/github.ts]
+import type { H3Event } from 'h3'
+
+export const cachedGHStars = defineCachedFunction(async (event: H3Event, repo: string) => {
+  const data: any = await $fetch(`https://api.github.com/repos/${repo}`)
+
+  return data.stargazers_count
+}, {
+  maxAge: 60 * 60,
+  name: 'ghStars',
+  getKey: (event: H3Event, repo: string) => repo
+})
+```
+```ts [api/stars/[...repo\\].ts]
+export default defineEventHandler(async (event) => {
+  const repo = event.context.params.repo
+  const stars = await cachedGHStars(event, repo).catch(() => 0)
+
+  return { repo, stars }
+})
+```
+::
+
+This way, the function will be able to keep the instance alive while the cache is being updated without slowing down the response to the client.
+
 ## Caching route rules
 
 This feature enables you to add caching routes based on a glob pattern directly in the main configuration file. This is especially useful to have a global cache strategy for a part of your application.
@@ -191,7 +223,6 @@ export default defineNuxtConfig({
 
 The `cachedEventHandler` and `cachedFunction` functions accept the following options:
 
-
 ::field-group
   ::field{name="base" type="string"}
     Name of the storage mountpoint to use for caching, default is `'cache'`.