docs: correct response.back() signature and behavior

[Bojun-Vvibe]

Repo: koajs/koa (⭐ 35000)
Type: docs
Files changed: 1
Lines: +11/-2

What

Fixes the response.back() API doc entry in docs/api/response.md. The
heading previously documented the method as response.back(url) and called the
parameter "url", but the actual implementation in lib/response.js accepts a
single optional alt parameter that is only used as a fallback when the
Referrer header is missing (or points to a different host). The doc is
updated to response.back([alt]), with a clearer description and a small
example matching the migration guide.

Why

The previous entry was misleading: readers could reasonably believe the
argument was the redirect target, when it is actually the fallback. This is
the same back([alt]) shape used in lib/response.js and the migration
guide (docs/migration-v2-to-v3.md), so the API reference is now consistent
with the rest of the docs and with the source.

Testing

Docs-only change. Verified the new wording matches the implementation in
lib/response.js (back (alt) — uses Referrer when same-host, otherwise
alt || '/') and the example from docs/migration-v2-to-v3.md.

Risk

Low — documentation only, no runtime behavior change.