docs: Middleware fetching is not cached (#80378)
Closes:
https://linear.app/vercel/issue/DOC-4751/docs-missing-middleware-caching-information-also-ts-types-issue
Fixes: https://github.com/vercel/next.js/issues/77900
Note that the TypeScript (should the TS plugin detect fetch
`options.next` used in middleware and error out?) issue is still
pending. It will be eventually handled.
