Document `Concurrent_tag::is_parallel`?

In PR #9274, I have added a value in `Parallel_tag` and `Sequential_tag`: https://github.com/CGAL/cgal/blob/ced13710423603b8acea8e806beaee3898705843/STL_Extension/include/CGAL/tags.h#L45-L47 That is the equivalent of `Tag::value` when `Tag` is a Boolean tag, like: - `CGAL::Boolean_tag<B>` (with the two specializations `CGAL::Tag_true`/`CGAL::Tag_false`), - `std::boolean_constant<B>` (with the two specializations `std::true_type`/`std::false_type`), - any specialization of a (_UnaryTypeTrait_)[https://en.cppreference.com/w/cpp/named_req/UnaryTypeTrait.html] `std::is_*something*<T>` from `<type_traits>` What do you think of that addition? Should we document it? If it were me, `Sequential_tag` and `Parallel_tag` would have been enumerators, instead of tag types, but we have to deal with what was chosen at the time (even at the time of CGAL 4.3, when the base C++ standard for CGAL was C++03, enums were allowed as non-type template parameters).