Document better the differences between sbws and torflow scaling

There's already some documentation at https://sbws.readthedocs.io/en/latest/how_works.html#running-the-generator and https://sbws.readthedocs.io/en/latest/torflow_aggr.html, but that does not include the pid values that torflow uses.

Documenting this better would help to evaluate #40040 (moved).