I personally think it's neater to mark the class with an @internal PHPDoc tag rather than add a sub-namespace.
http://phpdoc.org/docs/latest/references/phpdoc/tags/interna...
http://phpdoc.org/docs/latest/references/phpdoc/tags/interna...
In terms of neatness and ease of use, I choose like I'd choose how to optimize code - I focus on the parts that get most use. Users of a library are hopefully way more than its maintainers, so it feels like it's worth slightly inconveniencing the maintainers in order to have an instantly clear public interface.