Documentation: Clarify precedence / remove duplicate 'num processors' settings in settings.yaml

[ghost]

In the example, there are two entries about the number of processors to use. One is on the top, and the other is under multiprocess_steps. It seems that the one under multiprocess_steps supercedes the one at the top of the settings.yaml, so it is not used at all in the code. It may be better to only allow one entry in the settings.yaml to avoid confusion, or add explanations in the documentation for these two settings.