Document url constraint in data fetching (#3677)

Co-authored-by: Elian ☕️ <hello@elian.codes>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

src/content/docs/en/guides/data-fetching.mdx

@@ -8,7 +8,9 @@ i18nReady: true
 
 ## `fetch()` in Astro
 
-All [Astro components](/en/core-concepts/astro-components/) have access to the [global `fetch()` function](https://developer.mozilla.org/en-US/docs/Web/API/fetch) in their component script to make HTTP requests to APIs. This fetch call will be executed at build time, and the data will be available to the component template for generating dynamic HTML. If [SSR](/en/guides/server-side-rendering/) mode is enabled, any fetch calls will be executed at runtime.
+All [Astro components](/en/core-concepts/astro-components/) have access to the [global `fetch()` function](https://developer.mozilla.org/en-US/docs/Web/API/fetch) in their component script to make HTTP requests to APIs using the full URL (e.g. https://example.com/api or `Astro.url + "/api"`).
+
+This fetch call will be executed at build time, and the data will be available to the component template for generating dynamic HTML. If [SSR](/en/guides/server-side-rendering/) mode is enabled, any fetch calls will be executed at runtime.
 
 💡 Take advantage of [**top-level await**](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#top_level_await) inside of your Astro component script.