Improve the documentation for QElapsedTimer::restart, include units

Specify that QElapsedTimer::restart returns milliseconds

Change-Id: I47d9ffde7b0f73c30b14d2ce8467ec0a553b58f8
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit 302254f90f41509b3d1111551296134cd76a3db5)
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qelapsedtimer_generic.cpp

@@ -81,7 +81,8 @@ void QElapsedTimer::start() noexcept
 }
 
 /*!
-    Restarts the timer and returns the time elapsed since the previous start.
+    Restarts the timer and returns the number of milliseconds elapsed since
+    the previous start.
     This function is equivalent to obtaining the elapsed time with elapsed()
     and then starting the timer again with start(), but it does so in one
     single operation, avoiding the need to obtain the clock value twice.