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>