[ofiwg] HMEM requirements

[Hefty, Sean]

For those tracking the HMEM discussions in the OFIWG, with notes also captured in github here: 

https://github.com/ofiwg/libfabric/discussions/6535

I've created a patch that attempts to document the requirements for using HMEM in this pull request:

https://github.com/ofiwg/libfabric/pull/6629/files

The updates describe the behavior needed by providers and the semantics viewed by the application.  The updates should be capturing the existing, but assumed semantics and expectations of the API.

If there are any comments, please add them directly to the pull request to make it easier to track them.

- Sean