document VNI usage scenarios for Slingshot networks

[carns]

We need to carefully document VNI usage scenarios in a way that's portable across platform configurations and schedulers.

This impacts under what situations processes that are launched separately (either within the same job allocation or across job allocations) are able to communicate.

It varies based on whether a given platform has the default service vni enabled for users, whether vnis are allocated for job allocations or not, whether new vnis are allocated by launcher or not, and which scheduler/launcher combination the system is using. Some cases require consideration of launcher arguments, some cases require consideration of Mercury environment variables, and some cases will require updated DRC support.

Documentation should explain the concepts and describe how to determine the platform configuration in a general way rather than just providing per-platform recipes. Any per platform-specific instructions can be placed in the platform-configurations repo.