docs: explain about the conditional default metadataBase on vercel de… (#65583)
Explain more details about default `metadataBase` (where `metadataBase`
is not defined) when it's on Vercel deployment
x-ref: https://github.com/vercel/next.js/pull/65089
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Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>