Doc: viewing item hierarchy while debugging Qt Quick 2

When debugging Qt Quick 1, you right-click to open a
context menu, but for Qt Quick 2, it has been changed
to double-clicking to cycle through the item stack.

Task-number: QTCREATORBUG-13617
Change-Id: I3b36c76a54a24cbd102cd8ffa2bf5da2d12bef26
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>