Graph.discrete_demographic_events() should document pulse ordering.

[grahamgower]

Current behaviour of Graph.discrete_demographic_events() is to return the pulses list as is. This means the pulse order is maintained. This ordering is important when multiple pulses have the same time value, and we should document that the pulse order is maintained.

Compared to the other discrete events, the pulses list is not constructed on the fly, so we should also document that the Graph.pulses list is returned verbatim and shouldn't be modified. (Or maybe we should copy the list before returning it instead?)