Fix Apache functions return types to reflect the stubs

[scoopandrun]

The stubs for Apache functions apache_request_headers, getallheaders and apache_response_headers are as follows:

<?php

// https://github.com/php/php-src/blob/PHP-8.3.12/sapi/apache2handler/php_functions.stub.php
// https://github.com/php/php-src/blob/PHP-8.3.12/sapi/cgi/cgi_main.stub.php

function apache_request_headers(): array {}

/** @alias apache_request_headers */
function getallheaders(): array {}

function apache_response_headers(): array {}

However, the return types in the docs wrongly include false as a return type for those functions.
There even are some discrepancies within the same page between the Description content and the Return Values section.

This PR aims to fix these inconsistencies.

[Girgias]

It seems like it used to be able to return false in PHP 5, thanks for spotting this!