docs: link updateTag from cacheTag reference (#92379)
### What?
Add [`updateTag`](/docs/app/api-reference/functions/updateTag) to the
`cacheTag` API page: a **Related** link and a **Good to know** bullet
that explains when to use it (read-your-own-writes / immediate expiry)
and points to [Differences from
`revalidateTag`](https://nextjs.org/docs/app/api-reference/functions/updateTag#differences-from-revalidatetag).
### Why?
The bundled `cacheTag` doc only walked through `revalidateTag`, so the
two-API invalidation model was easy to miss. This keeps existing
`revalidateTag` wording and examples unchanged and adds a minimal
pointer to `updateTag` without duplicating its full reference.
### How?
- `related.links`: include `app/api-reference/functions/updateTag`.
- `## Good to know`: one imperative bullet + deep link to the comparison
section on the `updateTag` page.
## PR checklist (Improving Documentation)
- Prettier/ESLint ran via lint-staged on commit; run `pnpm prettier-fix`
if needed.
- Follow the docs contribution guide:
https://nextjs.org/docs/community/contribution-guide
Made with [Cursor](https://cursor.com)
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
