docs: mention the building variable to execute non-prerenderable code in server handle (#13153)

* Update 20-hooks.md

* Update documentation/docs/30-advanced/20-hooks.md

---------

Co-authored-by: Elliott Johnson <[email protected]>

Author: eltigerchino

documentation/docs/30-advanced/20-hooks.md

@@ -39,6 +39,8 @@ export async function handle({ event, resolve }) {
 
 If unimplemented, defaults to `({ event, resolve }) => resolve(event)`.
 
+During prerendering, SvelteKit crawls your pages for links and renders each route it finds. Rendering the route invokes the `handle` function (and all other route dependencies, like `load`). If you need to exclude some code from running during this phase, check that the app is not [`building`]($app-environment#building) beforehand.
+
 ### locals
 
 To add custom data to the request, which is passed to handlers in `+server.js` and server `load` functions, populate the `event.locals` object, as shown below.