MC: make it clear that we only have the info about the ready-to-run actors in a given...

MC: make it clear that we only have the info about the ready-to-run actors in a given mc::State

This is very similar to the exploration algorithm in Simix, but that's
still counter-intuitive when you try to understand the code.

Let's hope that writting it this way will help me not falling into
that trap anymore :)