Update Infos for MUST (MPI ERROR DETECTION)

Some more/correct information should be given. Give additional hints for correct usage on cluster

• Example should not be run on login node, because MPI does not work there
• It should use mustrun --must:mpiexec srun --must:np -n -n <ntasks> ./<my-program> instead to use srun instead of mpiexec and to run on the compute nodes
• additionally specify --must:stacktrace backward to get information on where the error occured. This might slow down execution a little bit.

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