docs: update usage about runtime config and environment variables

docs/content/migration/8.runtime-config.md

@@ -25,30 +25,39 @@ import { defineNuxtConfig } from 'nuxt'
 
 export default defineNuxtConfig({
   runtimeConfig: {
-    secretKey: '', // variable that can only be accessed on the server side
+    // Private config that is only available on the server
+    apiSecret: '123',
+    // Config within public will be also exposed to the client
     public: {
-      BASE_URL: process.env.BASE_URL || 'https://nuxtjs.org' // variable that can also be accessed on the client side
+      apiBase: '/api'
     }
   },
 })
 ```
 
 ```vue [pages/index.vue]
 <script setup>
-  const config = useRuntimeConfig().public
-  // instead of process.env.BASE_URL you will now access config.BASE_URL
+  const config = useRuntimeConfig()
+
+  // instead of process.env you will now access config.public.apiBase
+  console.log(config.public.apiBase)
 </script>
 ```
 
 ```ts [server/api/hello.ts]
-const config = useRuntimeConfig().public
+const config = useRuntimeConfig()
 
 export default (req, res) => {
-  // you can now access config.BASE_URL
-  return {
-    baseURL: config.BASE_URL
-  }
+  // In server, you can now access config.apiSecret, in addition to config.public
+  console.log(config.apiSecret)
+  console.log(config.public.apiBase)
 }
 ```
 
+```ini [.env]
+# Runtime config values are automatically replaced by matching environment variables at runtime
+NUXT_API_SECRET=api_secret_token
+NUXT_PUBLIC_API_BASE=https://nuxtjs.org
+```
+
 ::