Update documentation for InstructionToSignals to be much more explicit

[DanPuzzuoli]

With #164 , we're working towards making the InstructionToSignals class behave in a way more reflective of how schedules are implemented on real backends. As this behaviour is non-obvious and non-trivial, after #164, we should update the documentation for this class to be much more explicit about the rules its following to generate the samples used in simulation from schedules.