docs: warn directives must be registered on both server/client (#22627)

docs/2.guide/2.directory-structure/1.plugins.md

@@ -223,4 +223,8 @@ export default defineNuxtPlugin((nuxtApp) => {
 })
 ```
 
+::alert{type=warning}
+If you register a Vue directive, you _must_ register it on both client and server side unless you are only using it when rendering one side. If the directive only makes sense from a client side, you can always move it to `~/plugins/my-directive.client.ts` and provide a 'stub' directive for the server in `~/plugins/my-directive.server.ts`.
+::
+
 :ReadMore{link="https://vuejs.org/guide/reusability/custom-directives.html"}