Document behavior of nested state machines

Since QStateMachine is a QState, state machines can be nested. However, the semantics of such nesting is not documented anywhere – in particular, that a nested state machine is seen as an atomic state by the parent state machine. A section should be added to the "The State Machine Framework" page, based on this blog post: http://labs.qt.nokia.com/2009/07/23/qstatemachines-a-state-too/