docs: improve extend messages hooking (nuxt-modules#1870)

docs/content/2.guide/13.extend-messages.md

@@ -38,6 +38,23 @@ export default defineNuxtModule({
 
 Now the project has access to new messages and can use them through `$t('my-module-exemple.hello')`.
 
+::alert{type="info"}
+
+The custom module that is use `i18n:extend-messages` hook should be inserted before nuxt i18n module.
+
+```ts {}[nuxt.config.ts]
+import CustomModule from './custom' // import your custom module
+
+export default defineNuxtConfig({
+  modules: [
+    CustomModule,
+    '@nuxtjs/i18n',
+  ],
+})
+```
+
+::
+
 ::alert
 
 Because module's messages are merged with the project's ones, it's safer to prefix them.