Document Cylc ports better #850
Description
We currently document the default port range, in the global config reference.
Somewhere (Installation section?) we should also document:
- Schedulers bind two ports each
- What happens if no more ports are available, on a host
- TCP communications is uni-directional: from where Cylc commands run, back to the run host
- (this typically means compute nodes only need ports open TO run hosts, not the other way around).