docs: add about setCookie() prefix details

[kenjis]

Description

• update set_cookie()
• update Response::setCookie()

Ref #6009 (comment)

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

[kenjis]

The current behavior of cookie prefix:

If set to '', the default value from app/Config/Cookie.php will be used.

Is this a bug?
--> No, at least CI3 does so.

[kenjis]

On CI3,

$config['cookie_prefix']	= 'ci3_';

	$cookie = [
		'name'          => 'The Cookie Name',
		'value'         => 'The Value',
		'prefix'        => null,
	];
	$this->input->set_cookie($cookie);

Results:

prefix	cookie name
'prefix' => null	"ci3_The Cookie Name"
'prefix' => ''	"ci3_The Cookie Name"
'prefix' => 'prefix'	"prefixThe Cookie Name"
'prefix' => '0'	"0The Cookie Name"

It seems there is no way to remove the cookie prefix.

Ref, https://codeigniter.com/userguide3/libraries/input.html#CI_Input::set_cookie

[MGatner]

This surprised me but does seem to be intentional. I'm going to label this to revisit for version 5, because I don't think this is ideal.