Document events dispatched during preloading

Documents [@hotwired/turbo#1034][]

Document the `turbo:before-fetch-request` and
`turbo:before-fetch-response` events dispatched during `<a>` preloading.

[@hotwired/turbo#1034]: hotwired/turbo#1034

_source/handbook/02_drive.md

@@ -310,5 +310,9 @@ Preload links into Turbo Drive's cache using `<a href="/" data-turbo-preload>Hom
 
 This will make page transitions feel lightning fast by providing a preview of a page even before the first visit. Use it to preload the most important pages in your application. Avoid over usage, as it will lead to loading content that is not needed.
 
+Note that preloaded `<a>` elements will dispatch [turbo:before-fetch-request](/reference/events) and [turbo:before-fetch-response](/reference/events). Determine whether a `turbo:before-fetch-request` event is a preload request by checking the [Sec-Purpose:][] header (accessible from the `event.detail.fetchOptions.headers["Sec-Purpose"]` property).
+
 It also dovetails nicely with pages that leverage [Eager-Loading Frames](/reference/frames#eager-loaded-frame) or [Lazy-Loading Frames](/reference/frames#lazy-loaded-frame). As you can preload the structure of the page and show the user a meaningful loading state while the interesting content loads.
 <br><br>
+
+[Sec-Purpose:]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-Purpose#prefetch