flux-sched: describe ways to specify qmanager's queueing policies and behavior for system instance

[dongahn]

For the system instance, we will need backfilling policies instead of FCFS and other ways to tune the qmanager parameters. Minimum TOML support is there for qmanager and we just need to document some key ones to admin guide.

[garlick]

We added a config example to the admin guide, and also a reference to fluxion's man pages.