Improve AttributeProxy documentation for subscribe_event methods

[bourtemb]

The documentation of AttributeProxy subscribe_event methods should be improved in order to precise that the AttributeProxy object and the callback object passed when these methods are invoked should be alive until the events are no longer requested (until unsubscribe_event() is called) because only the pointer to these objects is stored and used by the event machinery.
This is already documented in DeviceProxy::subscribe_event() method but not yet in AttributeProxy::subscribe_event() methods.