QTimer: Add \note in documentation

Keeping the event loop busy with a zero-timer like
that is bound to cause trouble and highly erratic behavior of the UI.

Fixes: QTBUG-96869
Pick-to: 6.1 6.0 5.15
Change-Id: Idf02a7a7e0689c59e1223610a6525262ead56d8d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qtimer.cpp

@@ -253,6 +253,8 @@ void QTimer::start()
 
     If \l singleShot is true, the timer will be activated only once.
 
+    \note   Keeping the event loop busy with a zero-timer is bound to
+            cause trouble and highly erratic behavior of the UI.
 */
 void QTimer::start(int msec)
 {