QElapsedTimer: document Duration and TimePoint typedefs

Fixes: QTBUG-116876
Pick-to: 6.6
Change-Id: I71215d9f9e3caf0c50635a86c65c94a6f097cbe0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qelapsedtimer.cpp

@@ -201,6 +201,16 @@ bool QElapsedTimer::isMonotonic() noexcept
     return true;
 }
 
+/*!
+    \typealias QElapsedTimer::Duration
+    Synonym for \c std::chrono::nanoseconds.
+*/
+
+/*!
+    \typealias QElapsedTimer::TimePoint
+    Synonym for \c {std::chrono::time_point<std::chrono::steady_clock, Duration>}.
+*/
+
 /*!
     Starts this timer. Once started, a timer value can be checked with elapsed() or msecsSinceReference().