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 used dates here because I had problems with chronology. I'm not sure how to write that archived versions have more PHP4-specific informations than our current manual. Or do you suggest to remove this sentence completely?