docs: explain about the conditional default metadataBase on vercel de… (

vercel#65583)

Explain more details about default `metadataBase` (where `metadataBase`
is not defined) when it's on Vercel deployment

x-ref: vercel#65089

---------

Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>

docs/02-app/02-api-reference/04-functions/generate-metadata.mdx

@@ -430,8 +430,14 @@ export const metadata = {
 
 If not configured, `metadataBase` has a **default value**.
 
-- When [`VERCEL_PROJECT_PRODUCTION_URL`](https://vercel.com/docs/concepts/projects/environment-variables/system-environment-variables) or [`VERCEL_URL`](https://vercel.com/docs/concepts/projects/environment-variables/system-environment-variables) is detected, it will be used as the **default value**, otherwise it falls back to `http://localhost:${process.env.PORT || 3000}`. This allows Open Graph images to work on both preview and production deployments.
-- When overriding the default, we recommend using environment variables to compute the URL. This allows configuring a URL for local development, staging, and production environments.
+> On Vercel:
+>
+> - For production deployments, `VERCEL_PROJECT_PRODUCTION_URL` will be used.
+> - For preview deployments, `VERCEL_BRANCH_URL` will take priority, and fallback to `VERCEL_URL` if it's not present.
+>
+> If these values are present they will be used as the **default value** of `metadataBase`, otherwise it falls back to `http://localhost:${process.env.PORT || 3000}`. This allows Open Graph images to work on both local build and Vercel preview and production deployments. When overriding the default, we recommend using environment variables to compute the URL. This allows configuring a URL for local development, staging, and production environments.
+>
+> See more details about these environment variables in the [System Environment Variables](https://vercel.com/docs/concepts/projects/environment-variables/system-environment-variables) docs.
 
 #### URL Composition