docs: x-nextjs-stale-time header (#84901)
Let's add a note about this header.
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https://github.com/vercel/next.js/blob/3ae7ace09338bcde92ececb6b03a03760f302aed/packages/next/src/server/app-render/app-render.tsx#L2076-L2079
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https://github.com/vercel/next.js/blob/3ae7ace09338bcde92ececb6b03a03760f302aed/packages/next/src/client/components/segment-cache-impl/cache.ts#L262-L268
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https://github.com/vercel/next.js/blob/3ae7ace09338bcde92ececb6b03a03760f302aed/packages/next/src/client/components/segment-cache-impl/cache.ts#L318-L326
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Co-authored-by: Ismael <ismael@vercel.com>
