QTimer: fix calculation error in API docs

The range guaranteed by std¹ for nanosecond resolution is ±292 years,
not ±292 million.

¹ https://eel.is/c++draft/time.syn,
  https://en.cppreference.com/w/cpp/chrono/duration#Helper_types,
  std::cout << chrono::floor<years>(nanoseconds::max()) << '\n';

Since, in general, there is no integer type with ≥ 45 bits, but < 64
bits, the practical limit for milliseconds remains, indeed, ±292
million, so the QDateTime docs don't need fixing.

Change-Id: I5ae7f9706f147c6f490ba17b10cebe96426bbba2
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 051488c5faf5410e938978d4c1609c7a85074c57)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qtimer.cpp

@@ -78,7 +78,7 @@ QT_BEGIN_NAMESPACE
     The maximum interval QTimer supports is limited by the number of
     milliseconds that would fit in an \c int (which is around 24 days);
     whereas QChronoTimer stores its interval as \c std::chrono::nanoseconds
-    (which raises that limit to around 292 million years), that is, there is
+    (which raises that limit to ±292 years), that is, there is
     less chance of integer overflow with QChronoTimer.
 
     \section1 Accuracy and Timer Resolution