Improve description of interactive SLURM jobs

The current description of interactive SLURM jobs is misleading. It is not clear when to use salloc and when to use srun --pty bash and what the consequences are for each case. For example, it is not possible to start applications with srun within an srun --pty bash Job (unless you use --overlap, but I have no experience with that). On the other hand, srun --pty bash will give you a shell on a compute node, which salloc does not.

https://doc.zih.tu-dresden.de/jobs_and_resources/slurm/#interactive-jobs