srun --mpibind option is LLNL specific

[ryanday36]

There are a few places in the docs where example commands for launching flux under Slurm include a --mpibind=off flag. This flag refers to an LLNL specific plugin and will lead to errors for users at other sites.

examples:
https://flux-framework.readthedocs.io/en/latest/batch.html#launching-flux-in-slurm-s-batch-mode
https://flux-framework.readthedocs.io/en/latest/batch.html#fluxion-scheduler