[docs] Clarify usage of page.query (#2910)

* [docs] Clarify usage of `page.query`

* Update documentation/docs/03-loading.md

* Update documentation/docs/11-ssr-and-javascript.md

Co-authored-by: Rich Harris <hello@rich-harris.dev>

documentation/docs/11-ssr-and-javascript.md

@@ -72,6 +72,8 @@ The basic rule is this: for a page to be prerenderable, any two users hitting it
 
 Note that you can still prerender pages that load data based on the page's parameters, like our `src/routes/blog/[slug].svelte` example from earlier. The prerenderer will intercept requests made inside `load`, so the data served from `src/routes/blog/[slug].json.js` will also be captured.
 
+Accessing [`page.query`](#loading-input-page) during prerendering is forbidden. If you need to use it, ensure you are only doing so in the browser (for example in `onMount`).
+
 #### Route conflicts
 
 Because prerendering writes to the filesystem, it isn't possible to have two endpoints that would cause a directory and a file to have the same name. For example, `src/routes/foo/index.js` and `src/routes/foo/bar.js` would try to create `foo` and `foo/bar`, which is impossible.