docs: improve helpers.rst

[kenjis]

Description

• add note for case sensitive file system and namespace

See https://forum.codeigniter.com/showthread.php?tid=85977

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

[datamweb]

A long time ago, I had used namespace to write the helper, which caused the helper not to load.

<?php

declare(strict_types=1);

namespace App\Helpers;

if (! defined('myhelper')) {

    function myhelper() 
    {
        return 'XX';
    }
}

Maybe adding a note or warning would be nice. But I'm not sure if anyone other than me will repeat this funny mistake.

[kenjis]

Oh, it was loaded!

        helper('myhelper');
        \App\Helpers\myhelper(); // returns 'XX'

[datamweb]

\App\Helpers\myhelper(); // returns 'XX'

I used it like this.

helper('myhelper');
myhelper();

And I had this error:

Call to undefined function App\Controllers\myhelper() 

I understand how to use.
Thank you @kenjis .