docs: fix incorrect description on RedirectException (2)

[kenjis]

Description
Follow-up #7629
Ref #7628 (comment)

A complete URL will be converted to baseURL by base_url(). So it cannot be used with RedirectException since v4.0.0.

<?php

namespace App\Controllers;

use CodeIgniter\Router\Exceptions\RedirectException;

class Home extends BaseController
{
    public function index()
    {
        throw new RedirectException('https://codeigniter.com/foo/bar');
    }
}

$ curl -v http://localhost:8080/
*   Trying 127.0.0.1:8080...
* connect to 127.0.0.1 port 8080 failed: Connection refused
*   Trying [::1]:8080...
* Connected to localhost (::1) port 8080 (#0)
> GET / HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.87.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 302 Found
< Host: localhost:8080
< Date: Wed, 28 Jun 2023 01:33:53 GMT
< Connection: close
< X-Powered-By: PHP/8.1.20
< Cache-Control: no-store, max-age=0, no-cache
< Content-Type: text/html; charset=UTF-8
< Location: http://localhost:8080/foo/bar  <-- Look here
< 
* Closing connection 0

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide