Document --job "options", especially SLURM environment variables for controlling submission

[hattom]

As discussed on Slack, the current docs for job submission aren't so clear, especially w.r.t. how to control the SLURM submission (partition etc etc) with environment variables due to the lack of --job-partition=abc or similar.

[hattom]

While I'm at it, GC3Pie was deprecated in 5.0, but still present in the docs (and is described as the default backend)