docs: add information to preview mode (#534)

* docs: add information to preview mode

* [Update] Remove example

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

content/en/guides/features/live-preview.md

@@ -14,7 +14,7 @@ The preview mode will automatically refresh the page data as it uses `$nuxt.refr
 In order to activate live preview you will need to add the following plugin:
 
 ```js{}[plugins/preview.client.js]
-export default async function ({ query, enablePreview }) {
+export default function ({ query, enablePreview }) {
   if (query.preview) {
     enablePreview()
   }
@@ -64,17 +64,23 @@ enablePreview should be tested locally with yarn start and not yarn
 dev
 </base-alert>
 
-For pages that are not yet generated, SPA fallback will still call the API before showing the 404 page as these pages exist on the API but are not re-generated yet
+### Previewing pages that are not yet generated
 
-For pages that are not generated and don't exist on the API, for example draft content in your CMS that is not publicly available, you will need to use the validate hook:
+For pages that are not yet generated, SPA fallback will still call the API before showing the 404 page as these pages exist on the API but are not generated yet.
+
+If you have set a validate hook, you will probably need to modify it so that it doesn't redirect to the 404 page in preview mode.
 
 ```js
-async validate({ app, params, $preview }) {
-  if ($preview) {
+validate({ params, query }) {
+  if (query.preview) {
     return true
 }
 ```
 
+### Passing data to enablePreview
+
+You can pass data to the `enablePreview` function. That data will then be available on the `$preview` context helper and on `this.$preview`.
+
 ### What's next
 
 <base-alert type="next">