Warn users about PHP 4 manual status

Read http://news.php.net/php.doc/969385115 and [previous discussion](http://grokbase.com/t/php/php-doc/147da35f7q/proposal-of-removing-php4-references-from-the-manual/147da4mgrz) for more informations about this pull request.
I don't think we really need, but Philip suggested to mention this here and personally I don't see anything wrong in reminding people that PHP 4 is terribly old and unsupported, even if they exactly know it.