Added note about why Next.js used 307 and 308 status codes (#35325)

This PR looks to clear up some confusion users are having around our decision to use 307 and 308 redirects, instead of 301 and 302

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/api-reference/next.config.js/redirects.md

@@ -44,6 +44,9 @@ module.exports = {
 - `source` is the incoming request path pattern.
 - `destination` is the path you want to route to.
 - `permanent` `true` or `false` - if `true` will use the 308 status code which instructs clients/search engines to cache the redirect forever, if `false` will use the 307 status code which is temporary and is not cached.
+
+> **Why does Next.js use 307 and 308?** Traditionally a 302 was used for a temporary redirect, and a 301 for a permanent redirect, but many browsers changed the request method of the redirect to `GET`, regardless of the original method. For example, if the browser made a request to `POST /v1/users` which returned status code `302` with location `/v2/users`, the subsequent request might be `GET /v2/users` instead of the expected `POST /v2/users`. Next.js uses the 307 temporary redirect, and 308 permanent redirect status codes to explicitly preserve the request method used.
+
 - `basePath`: `false` or `undefined` - if false the basePath won't be included when matching, can be used for external rewrites only.
 - `locale`: `false` or `undefined` - whether the locale should not be included when matching.
 - `has` is an array of [has objects](#header-cookie-and-query-matching) with the `type`, `key` and `value` properties.