Improve the implementor's guide section on the Scheduler module (#1168)

* remove redundant resize instruction from guide

* amend past code pruning to fully cover acceptance period

* update guide again

* note on return value of `validation_code_at`

* rename execution cores to availability cores

* clarify terminology

* add ReplacementTimes to the guide

* fully decouple inclusion and scheduler

* clarify parathread scheduling substantially

* clarify validator group selection

Author: rphmeier