Documentation is missing `across_nodes`

[tygoetsch]

It looks like the documentation for the across_nodes scheduler option is missing from the ReadTheDocs for Pavilion2 as well as from the pav show config output. Here is the RTD page that looks like it should contain the documentation for across_nodes: https://pavilion2.readthedocs.io/en/latest/source/schedulers.html#pavilion.schedulers.SchedulerPlugin._kickoff