…ide (#92134)

### What?

Add a before/after migration snippet to the version 16 upgrade guide
showing the old single-argument `revalidateTag('posts')` and the new
required two-argument `revalidateTag('posts', 'max')` signature. Also
mention the TypeScript error users will see and the `updateTag`
alternative for immediate expiration.

### Why?

Users upgrading from Next.js 15 to 16 encounter `Expected 2 arguments,
but got 1` when using the old single-argument syntax. The upgrade guide
previously only showed the new code without showing the old pattern,
requiring users to search externally to understand what changed.

### How?

- Added a concise `// Before` / `// After` code block showing the
migration, matching the convention used elsewhere in the upgrade guide
(e.g. `next/legacy/image`)
- Added a note that the single-argument form produces a TypeScript error
- Added a note pointing to `updateTag` as the alternative for immediate
expiration
- Kept the original full example with stale-while-revalidate behavior
explanation

## PR checklist (Improving Documentation)

- Run `pnpm prettier-fix` to fix formatting issues before opening the PR
- Follow the docs contribution guide:
https://nextjs.org/docs/community/contribution-guide

It was discovered that one could let a remote source, safely-ish, load
local images, which has the benefit of providing, width, height, src,
and blur-* props which we could pass to next/image.

Closes:
https://linear.app/vercel/issue/DOC-6288/local-images-referenced-by-a-remote-source

Some corrections to the adapters docs.

- The example deploy script ran `cat .next/BUILD_ID` before `pnpm
build`. Under `set -euo pipefail` that kills the script immediately
since the file doesn't exist yet. Moved the build step first.

- `resolveRoutes` requires `URL` and `Headers` objects, not strings. The
example would cause TypeScript/runtime errors.

- Also documented the 6 return fields ~ other adapters might need to use
all of them for redirects, external rewrites, and middleware
short-circuiting.