docs: update runtime-config section (#4285)

Co-authored-by: Sébastien Chopin <seb@nuxtjs.com>

docs/content/2.guide/2.features/10.runtime-config.md

@@ -1,25 +1,34 @@
 # Runtime Config
 
-Nuxt provides an API to define the runtime config within your application and API routes.
+Nuxt provides a runtime config API to expose config within your application and server routes with the ability to update them at runtime using environment variables.
 
 ## Exposing runtime config
 
-To expose config and environment variables to the rest of your app, you will need to define runtime configuration in your `nuxt.config` file, using either the [`runtimeConfig` options](/guide/directory-structure/nuxt.config#runtimeconfig) (based on whether you want it to be accessible on the client-side part of your app or not).
+To expose config and environment variables to the rest of your app, you will need to define runtime configuration in your `nuxt.config` file, using the [`runtimeConfig` option](/guide/directory-structure/nuxt.config#runtimeconfig).
 
 **Example:**
 
 ```ts [nuxt.config.ts]
 export default defineNuxtConfig({
   runtimeConfig: {
-    API_SECRET: '123', // the private keys which should not be exposed to public
+    // The private keys which are only available within server-side
+    apiSecret: '123',
+    // Keys within public, will be also exposed to the client-side
     public: {
-      API_BASE: '/api',
+      apiBase: '/api'
+    }
   }
-  },
 })
 ```
 
-When adding `API_BASE` to the `runtimeConfig.public`, Nuxt adds it to the pages' payload. This way we can universally access `API_BASE` in both server and browser.
+When adding `apiBase` to the `runtimeConfig.public`, Nuxt adds it to each page payload. We can universally access `apiBase` in both server and browser.
+
+```js
+const runtimeConfig = useRuntimeConfig()
+
+console.log(runtimeConfig.apiSecret)
+console.log(runtimeConfig.public.apiBase)
+```
 
 ### Environment Variables
 
@@ -28,50 +37,54 @@ Nuxt CLI has built-in [dotenv](https://github.com/motdotla/dotenv) support.
 
 In addition to any process environment variables, if you have a `.env` file in your project root directory, it will be automatically loaded into `process.env` and accessible within your `nuxt.config` file and modules.
 
+Runtime config values are automatically replaced by matching environment variables at runtime.
+
 **Example:**
 
 ```sh [.env]
-BASE_URL=https://nuxtjs.org
-API_SECRET=api_secret_token
+NUXT_API_SECRET=api_secret_token
+NUXT_PUBLIC_BASE_URL=https://nuxtjs.org
 ```
 
 ```ts [nuxt.config.ts]
 export default defineNuxtConfig({
   runtimeConfig: {
-    API_SECRET: process.env.API_SECRET,
+    apiSecret: '',
     public: {
-      BASE_URL: process.env.BASE_URL,
+      apiBase: '', // Or a default value
     }
   },
 })
 ```
 
-**💡 Tip:** While it is not necessary, by using identical runtime config names as env variables, you can easily override them in production using platform environment variables.
-
 ## Accessing runtime config
 
 ### Vue app
 
 Within the Vue part of your Nuxt app, you will need to call `useRuntimeConfig()` to access the runtime config.
 
-**Note:** Behavior is different between client side and server side:
+**Note:** Behavior is different between the client-side and server-side:
 
-- On client side, only `runtimeConfig.public` is available and the object is both writable and reactive.
-- On server side, both `runtimeConfig.public` and `runtimeConfig` are merged and the object is read-only to avoid context sharing.
+- On the client-side, only keys in `public` are available, and the object is both writable and reactive.
+The entire runtime config is available on the server-side, but it is read-only to avoid context sharing.
 
 ```vue
 <template>
   <div>
-    <div>Token: {{ config.API_AUTH_TOKEN }}</div>
+    <div>Check developer console!</div>
   </div>
 </template>
 
 <script setup>
 const config = useRuntimeConfig()
+console.log('Runtime config:', config)
+if (process.server) {
+  console.log('API secret:', config.apiSecret)
+}
 </script>
 ```
 
-**🛑 Security note:** Never use example above if `API_AUTH_TOKEN` is a private config. Even if you use `runtimeConfig`, you still have to be careful that you do not expose such config to either payload or html!
+**🛑 Security note:** Be careful not to expose runtime config keys to the client-side by either rendering them or passing them to `useState`.
 
 ::alert{icon=👉}
 **`useRuntimeConfig` only works during `setup` or `Lifecycle Hooks`**.
@@ -81,46 +94,42 @@ const config = useRuntimeConfig()
 
 If you want to use the runtime config within any (custom) plugin, you can use `useRuntimeConfig()` inside of your `defineNuxtPlugin` function.
 
-For example:
+For Example:
 
 ```ts
 export default defineNuxtPlugin((nuxtApp) => {
   const config = useRuntimeConfig()
-
-  const url = process.server ? config.serverUrl : config.clientUrl
-
-  // Do something with url & isServer.
+  console.log('API base URL:', config.public.apiBase)
 });
 ```
 
-### API routes
+### Server Routes
 
-Within the API routes, you can access runtime config by directly importing from virtual `#nitro`.
+You can access runtime config within the server routes as well using `useRuntimeConfig`.
 
 ```ts
-import { useRuntimeConfig } from '#nitro'
-
-const config = useRuntimeConfig()
-
 export default async () => {
   const result = await $fetch('https://my.api.com/test', {
     headers: {
-      Authorization: `Bearer ${config.API_AUTH_TOKEN}`
+      Authorization: `Bearer ${useRuntimeConfig().apiSecret}`
     }
   })
   return result
 }
 ```
 
-### Typing runtime config
+### Manually Typing Runtime Config
 
-Currently it is possible to manually type your runtime config.
+Nuxt tries to automatically generate a typescript interface from provided runtime config using [unjs/untyped](https://github.com/unjs/untyped)
+
+It is also possible to type your runtime config manually:
 
 ```ts [index.d.ts]
 declare module '@nuxt/schema' {
-  interface runtimeConfig {
+  interface RuntimeConfig {
+    apiSecret: string
     public: {
-      testConfig: string
+      apiBase: string
     }
   }
 }