docs: add cache.varies docs for multi-tenant use case (#26197)

docs/3.api/2.composables/use-request-url.md

@@ -10,6 +10,12 @@ links:
 
 `useRequestURL` is a helper function that returns an [URL object](https://developer.mozilla.org/en-US/docs/Web/API/URL/URL) working on both server-side and client-side.
 
+::important
+When utilizing [Hybrid Rendering](/docs/guide/concepts/rendering#hybrid-rendering) with cache strategies, all incoming request headers are dropped when handling the cached responses via the [Nitro caching layer](https://nitro.unjs.io/guide/cache) (meaning `useRequestURL` will return `localhost` for the `host`).
+
+You can define the [`cache.varies` option](https://nitro.unjs.io/guide/cache#options) to specify headers that will be considered when caching and serving the responses, such as `host` and `x-forwarded-host` for multi-tenant environments.
+::
+
 ::code-group
 
 ```vue [pages/about.vue]