Update permanent redirect field documentation (#29557)

This explains what the permanent field does a bit more for redirects. 

## Documentation / Examples

- [x] Make sure the linting passes

x-ref: #29556

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

@@ -45,7 +45,7 @@ module.exports = {
 
 - `source` is the incoming request path pattern.
 - `destination` is the path you want to route to.
-- `permanent` if the redirect is permanent or not.
+- `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.
 - `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.