Draft: Update file finite-state-machines.md

Two open questions:

1. When is the init transition of a subgraph executed? During actor initialisation or when the state is first entered

2. Is the statemachine in the first image even valid (see below)? It does contain the same trigger from the transition point (top right) and from the "DoingThis" to "DoingThat" state. I guess this will produce validation errors. If not, what are the semantics of this construct?

One missing piece of documentation:

• Transitionpoints can be marked as "handler". In this case no exit and entry points are executed. Transitions originating from handler transition points should end in the same transition point.