Clarify that the issuer for a CommWait() action is that of the same actor
The `CommWait()` transition, in the manner that SimGrid
handles communications, can be said to be "issued" by either
the sender or the receiver that are a part of the communication
under consideration. We favor that which is run by the same actor;
indeed, we require it as this point unless otherwise noted.
If it is possible for a `CommWait` to wait on a communication
whose identity cannot be traced back to an event that already
executed by the same actor, the model would have to change
when computing extension sets for CommWait(), specifically
when checking whether or not that transition is enabled
at a prior state
