[Documentation] Cores and threads_per_core #999

The cores parameter is only useful when being used with mpi4py which is still a minority of the users of executorlib. So one suggestion would be to rename cores to mpi4py_cores and have the product of cores and threads_per_core accessible to the user under the variable name cores. But such a change would break the current examples and be confusing for users who are familiar with the current functionality, so it is not something I can do easily at the moment.

At the moment I extended the current documentation, to help clarify the confusion:
#998