Document QApplication::cursorFlashTime() can return -1

On Windows QApplication::cursorFlashTime() can return -1 if keyboard
blinking is disabled. Add documentation for that case.

Change-Id: I1e5d683573e55b045e2d9bd9804daf4be579fd02
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/widgets/kernel/qapplication.cpp

@@ -3648,6 +3648,9 @@ bool QApplication::keypadNavigationEnabled()
 
     We recommend that widgets do not cache this value as it may change at any
     time if the user changes the global desktop settings.
+
+    \note This property may hold a negative value, for instance if cursor
+    blinking is disabled.
 */
 void QApplication::setCursorFlashTime(int msecs)
 {