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 should say what is currently happening, rather I suggest we word this as though we already had removed the documentation to the archives.

Aside: "this explanation" isn't a very good link name, how about using the document title ("Documentation for PHP 4")?