Fix Traversable description

[mmalferov]

Following by:

https://bugs.php.net/bug.php?id=62609

and

php/php-src@e9ae581

It seems the implements Traversable construct for abstract classes was introduced only as of PHP 8.0.0, not in 7.4.0.

The following example works only as of PHP 8.0.0. Prior to 8.0.0, including [7.4.0, 7.4.33], it generates:

Fatal error: Class AbstractTraversable must implement interface Traversable as part of either Iterator or IteratorAggregate:

<?php

abstract class AbstractTraversable implements Traversable {}

class NonAbstractTraversable extends AbstractTraversable implements IteratorAggregate
{
    public function getIterator()
    {
        yield from ['foo', 'bar',];
    }
}

foreach (new NonAbstractTraversable() as $value) {
    echo $value, "\n";
}