InternalIterator class is reserved as of PHP 8.0.0

[mumumu]

We cannot instantiate or extend global scope class InternalIterator which is introduced as of PHP 8.0.0 ( php/php-src@ff19ec2 ) and seems to be reserved as internal iterator classname, but this fact is Undocumented.

https://3v4l.org/OKO6a

We should document it in appendices/migration80/incompatible.xml?

[cmb69]

See #896#899. I think documenting the new class in new-features.xml is sufficient.