C++20 comparison 6.7 remaining tasks

This task is created to collect things that were found during C++20 comparison chain review, but can be most likely addressed after Qt 6.7 FF.

In no specific order:

• Think how we can improve the helper functions from QTestPrivate. The problem is that if the test fails, QTestLib points us into the helper function, but not at the actual line of the test that calls this helper function. It becomes tricky if the test has multiple calls to the helper function.
• Fix documentation comments from https://codereview.qt-project.org/c/qt/qtbase/+/478199/46
• Try to apply https://codereview.qt-project.org/c/qt/qtbase/+/519927. We seem to have some disagreement about the proper ways of wrapping the macros. So let's postpone it until after FF.
• Adjusting the docs for the new relational operators. Ideally, qdoc will provide a new command for us.
• add C array overload of Qt::compareThreeWay
  • what about comparesEqual? We don't have that to specialize, and T[] == T[] will decay instead of iterating over the the elements
    • could just = delete it, people can use std::array as a drop-in replacement in most cases
      • will give us time to rethink this come Qt 6.8
• Introduce a negation operation for each ordering type so that the symmetric-order implementations for C++17 can simply return the negation of the reversed comparison, instead of needing to cast via strong_ordering.
• Make qfloat16 relational operators constexpr if QFLOAT16_IS_NATIVE.
• To be added...