QThread::wait: Clarify documentation for unsigned long overload

It is not immediately clear from the signature that the time parameter's
unit is miliseconds.

Pick-to: 6.2
Change-Id: Ifee3c6410b6b2352e75571cc53578a660aabb32d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthread.cpp

@@ -986,7 +986,10 @@ void QThread::setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)
 
 /*!
     \fn bool QThread::wait(unsigned long time)
+
     \overload
+    \a time is the time to wait in milliseconds.
+    If \a time is ULONG_MAX, then the wait will never timeout.
 */
 
 #if QT_CONFIG(thread)