QSemaphore: add a note to update to noexcept in Qt 7

The QSemaphore acquire & release functions, plus the constructor and
destructor, can be made noexcept on platforms that use futexes
(currently, Linux and Windows; likely macOS by the time we make 7.0). I
didn't make the change right now because the acquire and release
functions have narrow contracts on the int n values. I didn't make that
change now because it make a mess of the code, for little gain as this
class isn't used that often.

Plus, we may want to investigate whether we want to keep the multi-token
semaphore functionality around (something neither POSIX sem_t nor
std::counting_semaphore have), or split that off into a separate class
like QBasicMutex / QMutex / QRecursiveMutex was done.

Change-Id: Ieab617d69f3b4b54ab30fffd175b27813728f2ce
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

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