[ofiwg] improved p2p proposal

[Hefty, Sean]

Based on discussion in last week's ofiwg, I've created the following proposal for supporting GPU initiated data transfers:

https://github.com/ofiwg/libfabric/pull/7142

The proposal is mostly updates to the man pages to introduce a new feature called XPU triggers.  I plan to continue exploring the topic in next week's call.  I believe the proposal is implementation agnostic and can work with a variety of hardware designs.

I do not have a corresponding proposal for redirecting completions (NIC signals GPU); furthermore, I'm becoming less convinced that a HW agnostic interface will be possible.

- Sean