Document QElapsedTimer::Duration, QElapsedTimer::TimePoint

QElapsedTimer::Duration, QElapsedTimer::TimePoint are declared in code to be the same as std::chrono::nanoseconds, std::chrono::time_point<std::chrono::steady_clock, Duration>:

    using Duration = std::chrono::nanoseconds;
    using TimePoint = std::chrono::time_point<std::chrono::steady_clock, Duration>;

Anyhow, in the documentation that is documented only very indirectly: https://doc-snapshots.qt.io/qt6-6.6/qelapsedtimer.html . We should make this more explicit.