docs(use-cache): add invalidating section to use cache docs (#76958)
## Why?
The cacheTag mention [here](https://nextjs.org/docs/app/api-reference/directives/use-cache#revalidating) makes it seem like cacheTag does the "revalidating," but actually this is still revalidateTag's job.
- Related: https://github.com/vercel/next.js/issues/76581