Update namespace FAQ on overriding special constants

[haszi]

Removed ZEND_THREAD_SAFE and ZEND_DEBUG_BUILD from the section about defining namespaced special, built-in constants resulting in a fatal error as this is no longer the case for these two.

This code (https://3v4l.org/iMP1P)

<?php
namespace bar;
const ZEND_DEBUG_BUILD = true;
const ZEND_THREAD_SAFE = true;

var_dump(\ZEND_DEBUG_BUILD);
var_dump(\ZEND_THREAD_SAFE);

var_dump(ZEND_DEBUG_BUILD);
var_dump(ZEND_THREAD_SAFE);

no longer results in a fatal error but in this output

bool(false)
bool(false)
bool(true)
bool(true)

[Girgias]

Maybe should use the entities &null;, &false; and &true;

Also @Crell is or correct here or should it be and?